Saving such an image is a two-step procedure. First, the alpha mask must be ``glued'' to the main image as its alpha channel, after which the standard save procedure applies:

1. Choose Mask → Save To Alpha Channel.

2. Choose File → Save As (or Save Copy As).

Converting an existing truecolor image to palette-based or creating a new palette-based image involves essentially the same procedure:

1. Choose File → Open or New.

2. Choose Colors → Decrease Color Depth → 256 Colors.

Other depths are available, but most create the same size palette; indeed, the only other supported palette sizes in the output file are 2 and 16 colors. For an existing image, a dialog box will pop up offering different quantization methods (in the Palette section) and dithering methods (in the Reduction method section). Note that Nearest color means no dithering; Error diffusion is generally the nicest looking but slowest approach, sometimes known as Floyd-Steinberg or ``FS'' dithering in other programs. To add and view transparency, use the Colors menu again:

1. Choose Colors → Set Palette Transparency.

2. Choose Set the transparency value to the current background color.

3. Choose Colors → View Palette Transparency.

To set a color other than the background color as transparent, use the eyedropper tool to pick the color and find its index. Then, in place of the second step, select Set the transparency value to palette entry and enter the index value of the color.

Paint Shop Pro currently does not support gamma correction, even though it does provide a Monitor Gamma Adjustment window (via File → Preferences → Monitor Gamma) that could in theory be used to supply the appropriate information. PSP does add a modification-time chunk, but it is incorrectly written using the local time zone of the user rather than Universal Time as required by the PNG specification.

Text annotations, including those found in other file formats, are preserved and converted as needed. In addition, the user may add text chunks with the Title, Author, Copyright, and Description keywords via View → Image Information option. The program stores DOS-style line endings (both ``carriage return'' and ``line feed'' character codes) rather than following the PNG spec's recommendation to use Unix-style line endings (line-feed characters only).

With regard to file sizes, Paint Shop Pro always uses near-optimal compression and filtering settings on the image data. There is no option for faster compression, although PSP's own format is typically used for intermediate saves. The program's only major failing in this regard is that it always writes the maximum number of palette entries regardless of how many are used, and it doesn't reorder the palette so that the single transparent entry comes first, which would allow the remainder of the transparency chunk to be omitted. For a 50-color web icon with no transparency, this means the file will be 618 bytes larger than it should be, solely due to the overhead required to store a full 256 palette entries. With transparency, an average of 25 additional bytes would be wasted for this example, but the cost for true 256-color images may be as much as 255 bytes. As I've noted elsewhere, that can be a serious penalty for small images. In addition, PSP doesn't support writing three- or four-color images with 2 bits per pixel but instead will use 4 bits. Compression almost never makes up the difference; the output file will be roughly twice as large as it should be.

More information about Paint Shop Pro is available at Jasc's web site, http://www.jasc.com/psp.html.