PNG Optimization

“The type of a PNG image is defined in the IHDR image header. The image has a certain bit depth, up to 16 bits per sample, and a certain color type, from Grayscale to RGB+Alpha. If two PNG files of different types represent exactly the same image, each file can be regarded as a lossless transformation of the other. A lossless transformation can reduce the uncompressed stream, and such a transformation is named image reduction. In most cases, image reductions are capable of reducing the compressed stream (which is, in fact, our interest), as an indirect effect of reducing the size of the compressor’s input.

The possible image reductions are:

• Bit depth reduction
  The bit depth can be reduced to a minimum value that is acceptable for all samples. For example, if all sample values in a 16-bit image have the form (256+1)*n, (e.g. #0000, #2323, #FFFF), then the bit depth can be reduced to 8, and the new sample values will become n, (e.g. #00, #23, #FF).
• Color type reduction
  - If an RGB image has 256 distinct colors or less, it can be reencoded as a Palette image.
  - If an RGB or Palette image has only gray pixels, it can be reencoded as Grayscale.
  A color type reduction can also enable a bit depth reduction.
• Color palette reduction
  If the color palette contains redundant entries (i.e. duplicate entries that indicate the same RGB value) or sterile entries (i.e. entries that do not have a correspondent in the raw pixel data), these entries can be removed.
  A color palette reduction can also enable a bit depth reduction.
• Alpha channel reduction
  If all pixels in a Grayscale+Alpha or an RGB+Alpha image are fully opaque (i.e. all alpha components are equal to 2^bitdepth-1), or if the transparency information can be stored entirely in a (much cheaper) tRNS chunk, the alpha channel can be stripped.

There are, however, a few cases when some image type reductions do not necessarily lead to the reduction of the compressed stream. The PNG-Tech site contains experimental analyses of these possibilities; for example, see the article 8 bits per pixel in paletted images.

Interlacing, useful for a faster, progressive rendering, is another component of the PNG image type that affects compression. In an interlaced stream, the samples corresponding to neighboring pixels are stored far away, hence the data in it is less correlated and less compressible. Unlike JPEG, where interlacing may improve the compression slightly, the PNG interlacing degrades the compression significantly.”

Read the full article, learn more about .PNG Optimization.