Photoshop 5

Photoshop 5.0.2, available for Macintosh and 32-bit Windows, is the latest version of Adobe's flagship image editor, as of this writing. It supports colormapped, grayscale, and RGB PNGs at sample depths of 8 bits, and images optionally can be saved as interlaced. Alpha transparency is supported in grayscale and truecolor images, but there appears to be no way to add any sort of transparency to a palette-based image. Gamma and color correction are also supported, with one caveat; I'll come back to that shortly.

Photoshop 5 is modal, which is to say that images of a given type (e.g., RGB) remain of that type until explicitly converted to something else--a process that must occur before one attempts to save the image. For example, to convert an RGB or grayscale image to palette-based, follow this prescription:

1. Choose Image → Mode → Indexed Color, which pops up a dialog box.

2. Choose an appropriate Palette type (typically Adaptive).

3. Set the number of colors, either via the Color Depth selector or by entering the number explicitly in the Colors entry field.

4. Select a dithering method: None, Pattern, or Diffusion.

5. Choose Faster or Better color matching, and optionally check the Preserve Exact Colors box (disabled if no dithering).

6. Click the OK button.

The Color Depth selector rather disingenuously indicates bits per pixel, but it is actually nothing more than a shortcut for specifying a power-of-two number of colors--that is, 3 bits/pixel is 8 colors, 4 bits/pixel is 16, and so on. All settings result in 8-bit-per-pixel PNG files.

Note also that the Pattern dither type is known as ordered dithering in other contexts, while the Diffusion choice corresponds to Floyd-Steinberg or something similar. The latter generally looks much better, since human eyeballs are very good at noticing the regular patterns of an ordered dither.

Photoshop 5 includes a wizard for creating transparent images, but we'll step through the procedure manually. The key is not to rely on background transparency but instead to add a new layer representing alpha transparency. More specifically, given an image with or without background transparency, do the following to add an alpha channel to it:

1. In the Channels palette, click on the arrow at the upper right and select New Channel..., which pops up a dialog box.

2. In the Name: entry field, give the new channel a name (for example, Alpha) and click the OK button; the other fields can be left with their default values.

3. In the Channels palette again, return to the original RGB or grayscale channel.

4. Click on the Lasso tool (left side of tool palette, second from top).

5. In the Lasso Options tab of the tool palette, set the Feather radius to some value, perhaps 13.

6. Draw a loop around the face of the subject.

7. Do not invert the Lasso selection; instead go back to the Channels palette and select the alpha channel (the lassoed loop will still be visible on the blank channel).

8. Erase everything outside the loop via Edit → Clear.

9. Once again, return to the original RGB or grayscale channel via the Channels palette, and optionally click on the visibility box of the alpha channel to show its effects overlaid on the main image.

The preceding Lasso-related operations differ from those in every other image editor that I investigated, including Adobe's own ImageReady 1.0. Specifically, the requirement not to invert the selection in order to erase the outer part of the alpha channel seemed counterintuitive.[25]

[25] It should be noted, however, that I am by no means an expert with any of the image editors described here! It is entirely possible that there are settings or alternative approaches that conform more closely to the ``standard'' Lasso procedure used in the other programs.

Having added an appropriate alpha channel to the image, it may now be saved as a 16-bit gray+alpha or 32-bit RGBA PNG:

1. Choose File → Save a Copy..., which pops up the usual file dialog box.

2. Pick an appropriate directory and filename for the image, choose PNG as the format, and make certain the Exclude Alpha Channels checkbox is not checked.

3. Click the OK button, which triggers yet another dialog box.

4. Optionally create an interlaced PNG by selecting Adam7 as the interlacing type, and make sure the filter type is Adaptive for grayscale or truecolor images.

5. Click the OK button.

If transparency is only desired as an aid in creating the image, not as part of the actual file data, check the Flatten Image box in the Save dialog box.

Adobe made significant improvements to the overall handling of gamma and color correction in Photoshop 5, with explicit support for the new sRGB color space (see Chapter 10, "Gamma Correction and Precision Color") and a number of other standard color spaces, as well. Photoshop 5 also includes an option to enable Monitor Compensation (which requires that the monitor be specified correctly first), and it always saves gamma and color-correction information with PNG images.

Unfortunately, the gamma information PS5 saves in PNG images is wrong; it is always too small by a factor of two, resulting in images that display much too darkly. This is a significant problem, because it appears only to affect PNG images. In other words, one cannot simply make the appropriate compensation in Photoshop's RGB setup panel and forget about it; either PNG images will be written incorrectly, or all other image types will be read and written incorrectly. The only workaround within Photoshop 5 is to misadjust the display gamma setting just before saving a PNG image and to reset it just after saving. For example, in a typical Windows PC (or other sRGB display system) with a gamma value of 2.2, temporarily change the value to 1.1 in Photoshop's RGB Setup box (shown in Figure 4-1):

1. Choose File → Color Settings → RGB Setup...

2. Halve the Gamma: value (i.e., if it was 2.2, change it to 1.1).

3. Click the OK button.