The PNG Guide is an eBook based on Greg Roelofs' book, originally published by O'Reilly.

Image Editors

To create a PNG image from scratch, one needs an image editor that understands PNGs. But there are many levels of understanding, and only a handful of editors exercise PNG's most interesting features. Here is a list of the support one would like to see in the ideal image editor:

  • Basic image types: RGB, grayscale, and palette-based

  • Images with fewer than 256 colors automatically saved as palette-based (or grayscale, if appropriate)

  • Option to quantize and dither images with many colors down to 256 or fewer

  • Simple transparency with any basic image type (i.e., single color marked as fully transparent)

  • Full alpha transparency (also known as alpha channel or alpha mask)

  • ``Cheap'' RGBA-palette transparency (i.e., where each palette entry has red, green, blue, and alpha components)

  • Option to quantize and dither full RGBA images down to RGBA-palette images

  • Option to enable interlacing

  • Gamma correction, including calibration of display system

  • Color correction: either chromaticity, sRGB, or full ICC profiles

  • Ability to read, modify, and write 16-bit grayscale or 48-bit RGB images without conversion to lower bit depth

  • Reasonable default compression settings: adaptive filtering turned on for all image types except palette-based; ``medium'' zlib compression level (say, between 3 and 7); unused palette entries omitted; if simple/cheap transparency, palette ordered so that opaque transparency entries can be omitted

  • Options for both fast saves and best (slowest) compression

  • Ability to preserve and store user-defined text information

Not every feature is vital, of course, and some users may want only a subset of these. But particularly when it comes to web design, one would like full support for gamma correction and for PNG's various transparency capabilities, preferably with an option for best (or at least good) compression. On the other hand, when it comes to compression, one does not want to be overwhelmed with the minutiae of PNG's many compression parameters, particularly when PNG-specific optimization products exist (one of which will be covered in Chapter 5, "Applications: Image Converters").

In this chapter, I look at five of the most popular image-editing applications in detail, explaining how to invoke PNG-specific features and pointing out the limitations of each product. Because PNG's transparency options are among its most promising web-related capabilities, and because I wish to provide a concrete demonstration of the similarities and differences between the various editing programs, I will return to the sample editing task of Chapter 1, "An Introduction to PNG", An Introduction to PNG--namely, the step-by-step procedure for creating a soft ``portrait-style'' transparency mask in an existing image. At the end of the chapter, I list a couple of dozen other editors with PNG support.

Last Update: 2010-Nov-26