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Contents:
1.1. Overview of Image Properties
1.2. What Is PNG Good For?
1.2.1. Alpha Channels
1.2.2. Gamma and Color Correction
1.2.3. Interlacing and Progressive Display
1.2.4. Compression
1.2.4.1. Compression filters
1.2.4.2. Compression oopers
1.2.5. Summary of Usage
1.3. Case Study of a PNG-Supporting Image Editor
1.3.1. PNG Feature Support in Fireworks
1.3.2. Invoking PNG Features in Fireworks
1.3.3. Analysis of Fireworks PNG Support
1.3.4. Concluding Thoughts on Fireworks
PNG,[1]short for ``Portable Network Graphics,' is a computer file format forstoring, transmitting, and displaying images. Similar to the GIF and TIFFimage formats--in fact, designed to replace them in many applications--PNGsupports lossless compression, transparency information, and a range of colordepths. PNG also supports more advanced features such as gamma correction anda standard color space for precise reproduction of image colors on a wide rangeof systems and embedded textual information for storing such things as a title,the author's name, and explicit copyright.
[1] PNG is officially pronounced ``ping' (at least in English) but neverspelled that way. Yes, this was a major topic of discussion during itsdesign, and it is explicitly noted in the specification. Believe it ornot, in November 1998 the issue once again came under discussion, thistime with greater emphasis on non-English pronunciation. Though the``three-letter' approach (i.e., P-N-G spoken asthree separate letters) was not approved for inclusion in the spec, it maybe considered an acceptable unofficial alternative.
1.1. Overview of Image Properties
Grayscale and truecolor images are simpler in concept; the bytes used byeach pixel correspond directly to shades of gray or to colors. In agrayscale image of a particular pixel depth, a 0 pixel usually(though not always) means black, while the maximum value at that depthcorresponds to white. Intermediate pixel values are smoothly interpolatedto shades of gray, though this is often not as straightforward as it mightsound--gamma correction, a way of adjusting for differences incomputer display systems, comes in here. I'll give a brief overview ofgamma correction later in this chapter, and I'll discuss it at length in Chapter 10, 'Gamma Correction and Precision Color',Gamma Correction and Precision Color Silkypix developer studio 8 1 20 – raw photo correction. ;for now, I'll merely note that it is a Good Thing, and image formats thatprovide support for it can be viewed on different platforms without appearingtoo light on one and too dark on another.
A truecolor image uses three separate values for each pixel,corresponding to shades of red, green, and blue. Such images are oftenalso referred to as RGB. In Chapter 8, 'PNG Basics', I'll talkabout human vision and the reasons why mixtures of just three colors canappear to reproduce all colors, or at least a sufficiently large percentageof them that one need not quibble over the difference. I'll also mentionsome common alternatives to the RGB color space. To beconsidered truly truecolor instead of merely ``high color,' an image must contain at least 8 bits for each of the three colors in eachpixel; thus, at a minimum, a truecolor image has a depth of 24 bits.
While we're on the subject of colormapped images, two other concepts are worthmentioning: quantization and dithering. Suppose one has a 24-bit truecolorimage, but it must be displayed on a 256-color, palette-based display.Since truecolor images typically use anywhere from 10,000 to 100,000colors, the conversion to a colormapped image will involve substituting manyof the color values with a much smaller range of colors. This process isknown as quantization. Because the resulting images have such a limitedpalette of colors available to them, they often are unable to represent finecolor gradients such as the different shades of blue seen in the sky or therange of facial tones in a softly lit portrait. One way around this is todither the image, which is a means of mixing pixels of the availablecolors together to give the appearance of other colors (though generally atthe cost of some sharpness). For example, a checkerboard pattern ofalternating red and yellow pixels might appear orange. This effect isperhaps best illustrated with an example. Figure 1-1 shows a truecolorphotograph (here rendered in grayscale) together with two 256-color versionsof the same image--one simply quantized to 256 colors and the other bothquantized and dithered. The insets give a magnified view of one region,showing the relative effects of the two procedures.
Figure 1-1:(a) Original, 24-bit image; (b) same image after quantization, and (c) after quantization and dithering.(Click on images for full-scale, color versions.) |
I'll round out our review of image properties and concepts witha quick look at compression. There are really only two flavors: losslessand lossy. Lossless compression preserves the exact image data downto the last bit, so that what you get out after uncompressing is exactly thesame as what you started with. In contrast, lossy compression throwsaway some of the data in return for much better compression ratios. Forphotographic images, the best lossless methods may only manage a factor oftwo or three in compression, whereas lossy methods typically achieve anywherefrom 8 to 25 times reduction with very little visible loss of quality.I'll discuss the details of compression, particularly the lossless variety,at greater length in Chapter 9, 'Compression and Filtering'.
Finally, in describing the advantages of PNG, I will necessarily compareit with some older image formats. Although there are literallyhundreds of different formats, we will be most concerned with just three:GIF, JPEG, and TIFF. GIF, short for the Graphics Interchange Format,and JPEG, short for the Joint Photographic Experts Group (which definedthe format), are both very common image types often seen on the Web. TIFF, on the other hand, short for Tagged Image File Format, isalmost never used on the Web but is quite popular as an output format fromscanners and as an intermediate ``save format' while editing images. I'lltouch on the properties of each of these formats as we go.
1.2. What Is PNG Good For?
GIF was the original cross-platform image format for the Web, and it is stilla good choice in many respects. But PNG was specifically designed to replaceGIF, and it has three main advantages over the older format: alpha channels(variable transparency), gamma correction, and two-dimensional interlacing(a method of displaying images at progressively higher levels of detail).PNG also compresses better than GIF in almost every case, but the differenceis generally only around 5% to 25%, which is (usually) not a large enough factor toencourage one to switch on that basis alone. One GIF feature that PNG doesnot try to reproduce is multiple-image support, especially animations;PNG was and is intended to be a single-image format only. A very PNG-likeextension format called MNG has been developed to address this limitation; itis discussed in Chapter 12, 'Multiple-Image Network Graphics'.
1.2.1. Alpha Channels
All three of the basic PNG image types--RGB, grayscale, andpalette-based--can have alpha information, but currently it's most oftenused with truecolor images. Instead of storing three bytes for every pixel,now four are required: red, green, blue, and alpha, or RGBA. The variabletransparency allows one to create special effects that will look goodon any background, whether light, dark, or patterned. Forexample, a photo-vignette effect can be created for a portrait by making acentral oval region fully opaque (i.e., for the face and shoulders of thesubject), the outer regions fully transparent, and a transition region thatvaries smoothly between the two extremes. When viewed with a web browsersuch as Acorn Browse or Arena, the portrait would fade smoothly to whitewhen viewed against a white background or smoothly to black if against a black background. Both cases are shown in Figure 1-2.
Figure 1-2:Portrait with an oval alpha mask (a) against a white background and (b) against a black background.(Click on images for full-scale versions.) |
This feature is especially important for the small web graphics that aretypically used on web pages, such as colored (circular) bullets and fancytext. To avoid the jagged artifacts that really stand out on such images,most applications support anti-aliasing, a method for creating theillusion of smooth curves on a rectangular grid of pixels by smoothly varyingthe pixels' colors. The problem with anti-aliasing in the absence of variabletransparency is that it must be done against a predetermined background color,typically either white or black. Reusing the same images on a differentbackground usually results in an unpleasant ``halo' effect, as showninFigure 1-3. The standard approach is to create separate images for each background color usedon a site, but this has negative implications both for the designer, who wastestime creating and maintaining multiple copies of each image, and for visitorsto the site, who must download those copies.
Figure 1-3:Gray text anti-aliased against a white background, displayed against both white and black backgrounds. |
Alpha blending, on the other hand, effectively uses transparency as aplaceholder for the background color. Fully transparent regions will inheritthe background color as is; fully opaque regions will show up as the foregroundimages. This is no different from the usual case, exemplified by transparentGIFs. But the anti-aliased regions in between the fully transparent and fullyopaque areas are no longer pre-mixed with an assumed background color;instead, they are partially transparent and can be mixed with whatever backgroundon which the image happens to be placed.
[2] As it happens, the same algorithm that allows one to quantize a 24-bittruecolor image down to an 8-bit palette image also allows one to reduce a32-bit RGBA image to an 8-bit palette-alpha image. So it's not really thattricky for programmers; it's just not how they're used to thinking aboutsuch things.
1.2.2. Gamma and Color Correction
Fortunately, a new proposal for operating systems and physical devices avoidsthe overhead of a complete ICC profile. Called sRGB, for Standard RGBcolor space, it defines just that: a standard, unified color space thatdevices can support, thereby allowing true color management with minimalfile overhead and no need for the user to wade through a complicated end-to-endcalibration procedure. As of January 1999, the sRGB proposal was in``Committee Draft for Voting,' and it should be approved as an internationalstandard[3]by mid-1999; conformant devices should start appearing shortly thereafter.PNG supports sRGB via a chunk called, logically enough, sRGB.
Gamma, chromaticity, and color management are described in more detail inChapter 10, 'Gamma Correction and Precision Color'; PNG's basic structure, including the means by which it can beofficially or unofficially extended, is covered in Chapter 8, 'PNG Basics' and Chapter 11, 'PNG Options and Extensions'.
1.2.3. Interlacing and Progressive Display
PNG's approach to interlacing is two-dimensional and involves nostretching at all on more than half of its passes. Even-numberedpasses are stretched, but only by a factor of two--similar to theeffect after GIF's third pass. Some applications display only theodd-numbered PNG passes, so their pixels always appear square. Inaddition, PNG's interlacing consists of seven passes, as opposed toGIF's four. This means that the user will see an overall impression of theimage after only one-sixty-fourth of the data has arrived, eight timesfaster than GIF.[4]In the time it takes GIF to display its first pass, PNG displays fourpasses--and keep in mind that PNG's fourth pass is only one-quarteras stretched as GIF's first pass, with ``pixels' that are basically2 × 4 blocks instead of 1 × 8. As a general rule, textembedded in an interlaced PNG image becomes readable roughly twice asfast as in the identical interlaced GIF, as shown in Figure 1-4. The rows show the respective appearance after one-sixty-fourth,one-thirty-second, one-sixteenth, one-eighth, one-fourth, half, andall of the data has arrived. The first column shows GIF interlacing;the others show PNG interlacing, rendered in various styles: standardblocky rendering, interpolated rendering, and sparse rendering,respectively. Note that the word Interlacing has roughly thesame readability in the fifth GIF row, the fourth blocky PNG row, andthe third interpolated PNG row. In other words, the GIF text takestwo to four times as long to become readable.
[4] I am implicitly assuming that one-sixty-fourth of the compressed data (thestuff that can be said to ``arrive') corresponds to one-sixty-fourth of theuncompressed image data (what the user actually sees). This is notquite true for either PNG or GIF, though the difference is likely to be smallin most cases--and other factors, such as network buffering, will tend towash out any differences that do exist. See Chapter 9, 'Compression and Filtering' for more details.
Figure 1-4:Comparison of GIF interlacing (far left), normal PNG interlacing (second from left), PNG with interpolation (second from right), and PNG with sparse display (far right).(Click on image for full-scale version.) |
1.2.4. Compression
PNG's compression is among the best that can be had without losing image data and without paying patent or other licensing fees.[5]Patents are primarily of concernto application developers, not end users, but the decision to throw awaysome of the information in an image is very much an end-user concern. Thisinformation loss generally happens in two ways: in the use of a lesser pixeldepth than is required to represent all of the colors in the image, and inthe actual compression method (hence ``lossy' compression).
PNG supports all three of the main image types discussed earlier:truecolor, grayscale, and palette-based. TIFF likewise supports all three;JPEG only the first two; and GIF only the third, although it can fakegrayscale by using a gray palette. Both GIF and PNGpalettes are limited to a maximum of 256 colors, which means that full-colorimages--which usually have tens of thousands or even hundreds of thousandsof colors--cannot be stored as GIFs or palette-based PNGs without loss.[6]On the other hand, an image that does fit into a 256-color palette requiresonly one byte per pixel, which leads to an immediate factor-of-three reductionin file size over a full RGB image before any ``real' compression is doneat all. This fact alone is an important issue for PNG images, since PNG allowsan image to be stored either way.
[6] Technically that's not quite true in the case of GIF; it supports theconcept of multiple subimages, each of which may have its own palette andmay be tiled side by side with other subimages to form a truecolor mosaic.This mode is not widely supported, however, particularly on 8-bit displays.Even where it is supported as intended by its proponents, it is an incrediblyinefficient way to store and display truecolor image data.
So let's assume that the image type has been decided; that brings us to thecompression method itself. Both GIF and PNG use completely losslesscompression engines, and all but the most recently specified forms of TIFF doso as well. Standard JPEG compression is always lossy, however, even at thehighest quality settings.[7]Because of this, JPEG images are usually three to tentimes smaller than the corresponding PNG or TIFF images. This makes JPEG avery appealing choice for the Web, where small file sizes are important, butJPEG's compression method can introduce visible artifacts such as blockiness,color shifts, and ``ringing' or ``echos' near image features with sharpedges. The upshot is that JPEG is a poor choice for intermediate saves duringediting, and for web use it is best suited to smoothly varying truecolorimages, especially photographic ones, at relatively high quality settings.It is not well suited to simple computer graphics, cartoons, and many typesof synthetic images. Figure C-3 in the color insert demonstrates this:notice the dirty (or ``noisy') appearance of the blue-on-white text, thefaint yellow spots above and below it, the darker blue spots in the upperhalf, and the hints of pink in the white-on-blue text.
[7] There are two forms of truly lossless JPEG, which are discussed briefly inChapter 8, 'PNG Basics', but currently they are almost universally unsupported. There isalso a relatively new TIFF variant that uses ordinary (lossy) JPEG compression,but it is likewise supported by very few applications.
Among the popular lossless image-compression engines, PNG's engine isdemonstrably the most effective--even leaving aside the issue ofprefiltering, which I'll discuss in the next section. TIFF's best classiccompression method and GIF's (only) method are both based on an algorithmknown as LZW (Lempel-Ziv-Welch), which is quite fast and was used in the Unix utilitycompress and in the early PC archiver ARC. PNG's method is calleddeflate, and it is used in the Unix utility gzip (whichsupplanted compress in the Unix world) and in PKZIP (which replacedARC in the early 1990s as the preeminent PC archiver). Unlike LZW, deflatesupports different levels of compression versus speed--a dial, if you will.At its lowest setting,[8]deflate is as fast as or faster than LZW and compresses roughly the same; at itshighest setting, deflate is considerably slower but achieves noticeably bettercompression. (Decompression speed is essentially unaffected by the compressionlevel, except insofar as a less compressed image may take more time to readfrom network or disk.) The deflate algorithm is described in more detail inChapter 9, 'Compression and Filtering'.
[8] Actually I'm referring to deflate's second-lowest compressionsetting (``level 1'); the very lowest setting (``level 0') isuncompressed. Sadly, the dial only goes to 9, not 11.
1.2.4.1. Compression filters
Filtering is also described in more detail in Chapter 9, 'Compression and Filtering'.
1.2.4.2. Compression oopers
A more detailed list of compression tips for both users andprogrammers is presented in Chapter 9, 'Compression and Filtering'.
1.2.5. Summary of Usage
Table 1-1 summarizes the sorts of tasks for which PNG, JPEG, GIF, and TIFF tendto be best suited; question marks indicate debatable entries. (Keep inmind that there are always exceptions, though.)
Table 1-1.Comparison of Typical Usage for Four Image Formats |
PNG | GIF | JPEG | TIFF |
Editing, palette image, fast saves | |||
Editing, truecolor image, fast saves | |||
``Final' edit, best compression | |||
Editing, maximal editor portability | ? | ? | ? |
Web, truecolor image, no transparency | |||
Web, palette image, no transparency | |||
Web, image with ``on/off' transparency | |||
Web, image with partial transparency | |||
Web, cross-platform color consistency | |||
Web, animation | |||
Web, maximal browser portability | ? | ||
Web, smallest possible images |
The choice of a web format depends almost entirely on what features arerequired in the image. Transparency automatically rules out JPEG; partialtransparency rules out GIF, as well. For animation, GIF is the only choice.For opaque, photographic images, JPEG is the only reasonable choice--itscompression can't be beat. The truly critical issue, however, is portabilityacross browsers. GIF and JPEG are relatively safe bets, but what about PNG?By late 1997, it was supported (at least minimally) in virtually allbrowsers; Microsoft's Internet Explorer 4.0 and Netscape's Navigator 4.04finally got native PNG support in October and November 1997, respectively.[9]But gamma correction was supported only by Internet Explorer, and PNGtransparency was almost unusable. At the time of this writing,Navigator 5.0 is still unreleased, and IE 5.0 for Windows is unchangedfrom version 4.0. But there are strong indications that the Big Twowill finally support both gamma and full alpha-channel transparency intheir next major releases.
[9] Most other web browsers have supported PNG natively since 1995 or 1996.
So the preferred approach for PNG images is simply to wrap an OBJECTtag around an old-style IMG tag, where the OBJECT refers tothe PNG and the IMG refers to a JPEG or GIF version of the same image.I'll provide some concrete examples of this in Chapter 2, 'Applications: WWW Browsers and Servers',Applications: WWW Browsers and Servers.Newer browsers that support both PNG and OBJECT will render the PNGin the outer OBJECT, ignoring the IMG tag. Older browsers willeither ignore OBJECT as an unknown tag or else parse it but recognizethat they cannot render the PNG; either way, they will use the GIFor JPEG from the inner IMG tag, or the text in the ALT attribute ifthey do not support images.
At least, that's the theory. The main problem with this approach is that noversion of Navigator or Internet Explorer up through the latest 4.x releaseshandles OBJECT tags correctly. Both browsers will attempt to find aplug-in to handle an OBJECT image; lacking that, they will eitherrender the inner IMG or fail entirely. I'll look at this in moredetail in Chapter 2, 'Applications: WWW Browsers and Servers'.
But plug-in oddities notwithstanding, the IMG-within-an-OBJECTapproach works moderately well now and will only get better as browsers improvetheir conformance with WWW standards and as the need for external PNG plug-insdiminishes. Indeed, most of the images on the Portable Network Graphics homesite are referenced in this manner. As for referring to PNG images directlyin old-style IMG tags, which is more commonly thought of as``using PNG on the Web'--that depends on the images and on the targetaudience. For example, the Acorn home site already uses PNG images in places;their audience is largely Acorn users, and Acorn Browse has perhaps the bestPNG support of any browser in the world. But sites targeted at the averageuser running Navigator or Internet Explorer must keep in mind that any givenrelease of the Big Two browsers achieves widespread use only after a year orso, and even then, a large percentage of users continue to useolder versions. From a PNG perspective, this means that late 1998 was aboutthe earliest it would have been reasonable to begin using IMG-tag PNGson general-purpose sites. Sites that would like to make use of PNG transparencyor gamma support will have to wait until about a year after the 5.0 releasesoccur, which presumably means sometime in the year 2000. (PNG as the ImageFormat of the New Millennium[10]has a nice ring to it, though.)
[10] That would be the millennium of four-digit years beginning with thenumeral ``2,' which, of course, is what everyone will be celebratingon New Year's Eve, 1999. (The Third Millennium is the one that startson January 1, 2001.)
1.3. Case Study of a PNG-Supporting Image Editor
Fireworks is an image editor with a feature set that rivals Adobe Photoshopin many ways, but with far more emphasis on web graphics and less on high-endprinting support. In this, it is closer to Adobe ImageReady, a web-specificapplication intended to tune image colors and optimize file sizes. I'llcome back to Photoshop and ImageReady in Chapter 4, 'Applications: Image Editors'.
1.3.1. PNG Feature Support in Fireworks
Fireworks 1.0 supports a good range of PNG features and image types, and ittruly shines in its handling of transparency--indeed, its native internalformat is 32-bit RGBA (truecolor with a full 8-bit alpha channel) for allimages, and it can save this format, too. In addition, ordinarysingle-color (GIF-like) transparency is supported in both palette-based andRGB image types, and PNG's unique ``RGBA palette' mode is also supported.Nor is this support limited to recognizing when an image contains 256 or fewercolor-transparency combinations; with a suitable choice of export options,Fireworks can (within limits) quantize and optionally dither even a truecolorimage with a nontrivial alpha channel to an 8-bit RGBA-palette image.
[11] A tight release schedule was the main reason for the lack of areal fix in version 2.0; Macromedia engineers were fully aware ofthe deficiencies in the workaround and are expected to addressthem in the next release.
As one would expect of a graphics application targeted at the Web,Fireworks doesn't preserve 16-bit samples, although it will read16-bit PNG images (for example, from a medical scan) and convert thesamples to 8 bits. Butler 4 3 1. Slightly more surprising is its lack of support fortrue grayscale PNGs; Fireworks saves these as palette-based files,with a palette composed entirely of grayscale entries. This is aperfectly valid type of PNG file, but the required palette adds up to780 bytes of unnecessary overhead, a distinct liability for icons andother tiny images. On the other hand, a palette-based grayscale imagewith transparency can include a colored palette entry to be used asthe background color, something that PNG does not support for truegrayscale files.
In addition to supporting PNG as an output format, Fireworks actuallyuses PNG as its native file format for day-to-day intermediatesaves. This is possible thanks to PNG's extensible ``chunk-based'design, which allows programs to incorporate application-specific datain a well-defined way. Macromedia has embraced this capability,defining at least four custom chunk types that hold various thingspertinent to the editor. Unfortunately, one of them (pRVW) violates thePNG naming rules by claiming to be an officially registered, publicchunk type, but this was an oversight and should be fixed in version2.0.
Although it is entirely possible to use the intermediate Fireworks PNG filesin other applications, including on the Web (in fact, one of the``frequently asked questions' on the Fireworks web site specifically mentionsNetscape, Internet Explorer, and Photoshop), they are not really appropriatefor such usage. One reason is that the native PNG format reflects Fireworks'sinternal storage format, which, as mentioned earlier, is 32-bit RGBA.Even if the image contains only two colors and no transparency, it is savedas a 32-bit PNG file. That certainly doesn't help the old compression ratioany, but the potential for expansion due to the image depth is oftenovershadowed by that due to the custom chunks, several of which are huge.[12]Thanks to these chunks (which are meaningless to any application butFireworks), the intermediate PNG files can easily be larger than acompletely uncompressed RGBA image would be.
[12] In a 590k tutorial image from Macromedia's web site, 230k is due to imagedata; 360k is due to custom chunks.
1.3.2. Invoking PNG Features in Fireworks
Because Fireworks's internal format is 32-bit (i.e., truecolor plus a fullalpha channel), working with transparency is as easy as opening an image and applying the Eraser tool to its background. For example, suppose youhave a photograph of someone and want to focus on the face by making everything else transparent, leaving behind an oval (or at least roundish)portrait shot with a soft border. There are several ways to accomplish this,but the following prescription is one of the simplest:
Open the original image (File → Open).
Pick the background image (Modify → Background Image).
Double-click on the Lasso tool (right side of tool palette, second from top).
In the Tool Options pop-up, pick Feather and a radius, perhaps 25.
Draw a loop around the face of the subject.
Invert the lasso selection so that the part outside the loop gets erased (Select → Inverse).
Erase everything outside the loop via Edit → Clear (or do so manually with the Eraser tool).
Note that the Lasso tool's feathering radius is subtly different from thatavailable via the Select menu. The latter is a smoothing factor forthe Lasso's boundaries/; in this example, with an inverted selectionso that the image's rectangular boundary is also lassoed, changing thevalue through the menu will round off the corners of the dashed Lassoboundary and may merge separated parts of it together. The feathering radiuson the Tool Options pop-up affects only the width of the partially transparentregion generated along the Lasso's boundary.
In any case, that's all there is to creating an image with transparency. Thenext step is to save it as a PNG file. As I just noted, the Save and SaveAs.. menu items save the complete Fireworks ``project,' retaining informationabout the objects in the image and the steps used to create them, at aconsiderable cost in file size. It is generally worthwhile to save a copythat way in case further editing is needed later. But for publishing theimage on the Web, it must be exported, and this is where it can beconverted into a palette-based image with or without transparency--or leftas a 32-bit RGBA image, but without all of the extra editing information included.
First let's consider the case of exporting the image as a full RGBAfile. Here are the available options in the Export dialog box:
Cookie 5 7 6 – protect your online privacy concerns. Format: PNG
Bit Depth: Millions +Alpha (32 bit)
Fireworks 1.0 provides no option to interlace the image, so thepreceding steps represent the complete list of possibilities for thiscase. Things get more interesting when it comes to palette-based (orindexed-color) images. Then one has the option ofchoosing either single-color transparency or the nicer RGBA-palettetransparency, in addition to a number of other palette-relatedoptions. Here are the options for the RGBA-palette case:
Format: PNG
Bit Depth: Indexed (8 bit) (this is the default)
Palette: WebSnap Adaptive (default) or Adaptive
Dither: Check on or off
Transparency: Alpha Channel
Interlaced: Checkbox may be checked but does nothing in version 1.0
Figure 1-5:Fireworks Export Preview window showing RGBA-palette options.(Click on image for full-scale version.) |
Note that the effects of the current options are reflected in the previewimage to the right (as in Figure 1-5), whichshows a limitation in Macromedia'soriginal implementation of RGBA-palette mode. In particular, only four levelsof alpha are used, two of which are either complete transparency or completeopacity (the other two represent one-third and two-thirds transparency), which resultsin very noticeable banding effects in Figure 1-6.
Figure 1-6:Example of Fireworks RGBA-palette image showing strong banding. |
The four-level approach works quite well for anti-aliasing (that is,preventing ``jaggies' on curved elements such as circles or text),which effectively involves a one-pixel-wide band of variabletransparency lying between regions of complete transparency andcomplete opacity. But the previous example uses a 25-pixel-widefeathering radius, and the two partial-transparency bands both show upextremely well and have sharply defined edges even if dithering isturned on. Unfortunately, that rather defeats the purpose of alphatransparency in this case; the 32-bit version is the onlyalternative. Fortunately this was one of the areas that got fixed inversion 2.0, and judging by one test image, the results arespectacular.
Very nearly the same procedure works if you want to save the image withsingle-color, GIF-like transparency; instead of picking Alpha Channelfrom the list of options in the Transparency pull-down box, this timepick Index Color. Doing so once will allocate a single palette entry,not used elsewhere in the image, to act as the fully transparent color. Astrange feature of version 1.0 is that the Transparency pull-downwill still indicate Alpha Channel the first time Index Color ischosen. Choosing it again will cause it to ``stick,' but at a cost: the entry chosen for transparency, which generally seems to be the last one(usually black), may now be used in the opaque parts of the image as wellas the transparent regions. It is not clear whether this is a bug or anintentional feature of some sort, but it is fully reproducible. Figure 1-7 shows an example.
Acorn 5 The Image Editor For Humans 5 3 1 For Powerlifting
Figure 1-7:Fireworks Export Preview after choosing Index Color transparency twice, showing transparency (white artifacts) in opaque regions.(Click on image for full-scale version.) |
As with transparent GIFs, single-color PNG transparency requires that theimage be displayed against a suitable background color--white, in ourexample--to look good. The opposite case, displaying againstblack, is shown in Figure 1-8.
Figure 1-8:Example of a Fireworks image with single-color transparency, displayed against the ``wrong' background. |
1.3.3. Analysis of Fireworks PNG Support
I should note a few caveats about the implementation of indexed-colorimages and transparency in Fireworks 1.0. For example, the dithercheckbox seems to have very little effect in any of the paletteexamples, and no effect at all on the alpha channel in RGBA images; infact, the export ``wizard' explicitly notes this and actuallyrecommends against its use. And the palette-size pull-down seems tohave been borrowed from the GIF user interface--it allows onlypower-of-two palette sizes (e.g., 64, 128, 256) even though PNG'spalette chunk can have any number of entries from 1 to 256. The finaljump is particularly abrupt; it may happen that 160 colors is theperfect trade-off between quality and image size, but such an imagewould have to be saved with either 128 or 256 colors.
Because Fireworks's internal format is 32-bit (i.e., truecolor plus a fullalpha channel), working with transparency is as easy as opening an image and applying the Eraser tool to its background. For example, suppose youhave a photograph of someone and want to focus on the face by making everything else transparent, leaving behind an oval (or at least roundish)portrait shot with a soft border. There are several ways to accomplish this,but the following prescription is one of the simplest:
Open the original image (File → Open).
Pick the background image (Modify → Background Image).
Double-click on the Lasso tool (right side of tool palette, second from top).
In the Tool Options pop-up, pick Feather and a radius, perhaps 25.
Draw a loop around the face of the subject.
Invert the lasso selection so that the part outside the loop gets erased (Select → Inverse).
Erase everything outside the loop via Edit → Clear (or do so manually with the Eraser tool).
Note that the Lasso tool's feathering radius is subtly different from thatavailable via the Select menu. The latter is a smoothing factor forthe Lasso's boundaries/; in this example, with an inverted selectionso that the image's rectangular boundary is also lassoed, changing thevalue through the menu will round off the corners of the dashed Lassoboundary and may merge separated parts of it together. The feathering radiuson the Tool Options pop-up affects only the width of the partially transparentregion generated along the Lasso's boundary.
In any case, that's all there is to creating an image with transparency. Thenext step is to save it as a PNG file. As I just noted, the Save and SaveAs.. menu items save the complete Fireworks ``project,' retaining informationabout the objects in the image and the steps used to create them, at aconsiderable cost in file size. It is generally worthwhile to save a copythat way in case further editing is needed later. But for publishing theimage on the Web, it must be exported, and this is where it can beconverted into a palette-based image with or without transparency--or leftas a 32-bit RGBA image, but without all of the extra editing information included.
First let's consider the case of exporting the image as a full RGBAfile. Here are the available options in the Export dialog box:
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Bit Depth: Millions +Alpha (32 bit)
Fireworks 1.0 provides no option to interlace the image, so thepreceding steps represent the complete list of possibilities for thiscase. Things get more interesting when it comes to palette-based (orindexed-color) images. Then one has the option ofchoosing either single-color transparency or the nicer RGBA-palettetransparency, in addition to a number of other palette-relatedoptions. Here are the options for the RGBA-palette case:
Format: PNG
Bit Depth: Indexed (8 bit) (this is the default)
Palette: WebSnap Adaptive (default) or Adaptive
Dither: Check on or off
Transparency: Alpha Channel
Interlaced: Checkbox may be checked but does nothing in version 1.0
Figure 1-5:Fireworks Export Preview window showing RGBA-palette options.(Click on image for full-scale version.) |
Note that the effects of the current options are reflected in the previewimage to the right (as in Figure 1-5), whichshows a limitation in Macromedia'soriginal implementation of RGBA-palette mode. In particular, only four levelsof alpha are used, two of which are either complete transparency or completeopacity (the other two represent one-third and two-thirds transparency), which resultsin very noticeable banding effects in Figure 1-6.
Figure 1-6:Example of Fireworks RGBA-palette image showing strong banding. |
The four-level approach works quite well for anti-aliasing (that is,preventing ``jaggies' on curved elements such as circles or text),which effectively involves a one-pixel-wide band of variabletransparency lying between regions of complete transparency andcomplete opacity. But the previous example uses a 25-pixel-widefeathering radius, and the two partial-transparency bands both show upextremely well and have sharply defined edges even if dithering isturned on. Unfortunately, that rather defeats the purpose of alphatransparency in this case; the 32-bit version is the onlyalternative. Fortunately this was one of the areas that got fixed inversion 2.0, and judging by one test image, the results arespectacular.
Very nearly the same procedure works if you want to save the image withsingle-color, GIF-like transparency; instead of picking Alpha Channelfrom the list of options in the Transparency pull-down box, this timepick Index Color. Doing so once will allocate a single palette entry,not used elsewhere in the image, to act as the fully transparent color. Astrange feature of version 1.0 is that the Transparency pull-downwill still indicate Alpha Channel the first time Index Color ischosen. Choosing it again will cause it to ``stick,' but at a cost: the entry chosen for transparency, which generally seems to be the last one(usually black), may now be used in the opaque parts of the image as wellas the transparent regions. It is not clear whether this is a bug or anintentional feature of some sort, but it is fully reproducible. Figure 1-7 shows an example.
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Figure 1-7:Fireworks Export Preview after choosing Index Color transparency twice, showing transparency (white artifacts) in opaque regions.(Click on image for full-scale version.) |
As with transparent GIFs, single-color PNG transparency requires that theimage be displayed against a suitable background color--white, in ourexample--to look good. The opposite case, displaying againstblack, is shown in Figure 1-8.
Figure 1-8:Example of a Fireworks image with single-color transparency, displayed against the ``wrong' background. |
1.3.3. Analysis of Fireworks PNG Support
I should note a few caveats about the implementation of indexed-colorimages and transparency in Fireworks 1.0. For example, the dithercheckbox seems to have very little effect in any of the paletteexamples, and no effect at all on the alpha channel in RGBA images; infact, the export ``wizard' explicitly notes this and actuallyrecommends against its use. And the palette-size pull-down seems tohave been borrowed from the GIF user interface--it allows onlypower-of-two palette sizes (e.g., 64, 128, 256) even though PNG'spalette chunk can have any number of entries from 1 to 256. The finaljump is particularly abrupt; it may happen that 160 colors is theperfect trade-off between quality and image size, but such an imagewould have to be saved with either 128 or 256 colors.
With regard to transparency, the placement of transparententries in the Export window's palette view is directly reflected in the PNGfile's palette, whether Alpha Channel or Index Color is selected. This is regrettable, since the transparent colors are scattered all over thepalette in the alpha case. The single-color case is even worse--thetransparent color is the very last entry in the palette. As noted earlier,the preferred approach is to put all of the transparent entries at thebeginning of the palette so that the redundant information about opaquecolors can be eliminated from the transparency chunk. For a photographicimage saved in palette format with single-color transparency, the cost is127 or 255 bytes of wasted space.
PNG also supports a single-color (or single-shade), ``cheap' transparencymode thatworks with truecolor and grayscale images and avoids the need for a fullalpha channel, but there is no way to invoke this feature in Fireworks. Thelack of any grayscale support other than palette-based means that a grayimage with an alpha channel must be saved either as RGBA, doubling its size,or as an indexed image with transparent palette entries, generally with somedata loss. (The loss comes about because there are only 256 possiblegray+alpha combinations in palette mode, whereas a full gray+alpha imagesupports up to 65,536 combinations.) There is also no support for a PNGbackground-color chunk.
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Images that already have transparency are preserved quite well (recall that everything is stored internally as 32-bit RGBA), and Fireworksprovides quite a number of options beyond what described earlier for addingor modifying transparency. One in particular that could be used for unsharpmasking and other special effects is invoked via the Xtras menu. Withthe background image selected, choose Other → Convert toAlpha, which first converts the image to grayscale and then to an alphamask. The lightest parts of the image become the most transparent, whilethe black parts remain opaque.
Fireworks's compression is reasonably good. Even though there are no useroptions to adjust the compression level, the default level is a good trade-offbetween speed and size. Truecolor images tend to be compressed within a fewpercent of the best possible size, while indexed-color images may see upwardof 15% improvement when run through an optimization tool such as pngcrush(discussed in Chapter 5, 'Applications: Image Converters').
Fireworks also does a good job preserving PNG text annotations, albeit witha quirk: it removes all of the line breaks (``newlines'), for some reason.(Oddly enough, GIF and JPEG comments are not preserved.) The programadds its own Software text chunk; as one might expect, any incoming imagethat already includes such a chunk will find it replaced. This is a minorbreach of PNG etiquette, but one that helps keep tiny image files fromgetting noticeably bigger because of text comments.
Fireworks 1.0 also adds a Creation Time text chunk to most images itexports. This is not really a problem, per se; what is unusual is thatthe chunk's contents are invariably ``Thu, May 7, 1998'--a datethat has nothing to do with any of the images or even with the releaseof Fireworks 1.0. See alsoChapter 11, 'PNG Options and Extensions'for a discussion of why ``creation time' is a fuzzy concept.Version 2.0 was to have corrected this, replacing the Creation Time textchunk with PNG's officially defined timestamp chunk, tIME, but I did nothave a chance to verify that. The tIME chunk indicates the time of lastmodification, which is a more precisely defined concept and one that isappropriate for an image editor.
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As noted earlier, the ability to save interlaced PNG images willfirst be implemented as a global preference setting. As of January1999, the plan was for this to require editing version 2.0'spreferences file. Under Windows, this file is called FireworksPreferences.txt and is in the Fireworks installation directory(C:Program FilesMacromediaFireworks, by default); on theMacintosh, it is called Fireworks Preferences and is found in theSystem Folder:Preferences folder. Open the file in any texteditor and find the line:
Change this to the following to make all exported images interlaced:
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This change will take effect only after Fireworks 2.0 is restarted.Fortunately, later releases are expected to have a normal checkbox option.