GIF formats – diffusion, dither etc

Photoshop gif save options (from the top down), the first menu is the Colour Reduction Algorithm menu, it has the following options:

  • Perceptual Creates a colour table by favouring colours best perceived by the human eye
  • Selective This is usually the closest format to your original image, and is the default setting. It creates a colour table weighted toward large areas of colour.
  • Adaptive Uses only colours from the original image.
  • Restrictive Limits the palette to the 216 web safe colours.
  • Custom A colour table made by you the user. For example, one of my saved Custom palettes is for when I use black type in a transparent gif there are sometimes rough white edges to the letters – I set the Colour Reduction to greyscale then delete all the lighter colours.

The pull down Colours menu lets you choose the number of colours in the gif colour table.

Next down is the Dithering Algorithm menu, this simulates the colours in the original image by mixing things up a bit. If you drop dither to 0% you’ll get bands of blocky colour, at 100% it’ll look kind of pointillist. You’ve got three ways to dither:

  • Diffusion Randomizes colours in adjacent pixels
  • Pattern A half-tone square pattern
  • Noise Randomizes colours like Diffusion, but without spreading out into adjacent areas

Next is Transparency. If you have transparency, you have a range of options as to how the semi-transparent pixels around the opaque pixels of your image behave. This is easiest explained by example, if say you wanted a transparent gif of black type and were going to use it on top of a red background, you could choose a red foreground colour, select “Foreground Colour” in the Matte pull down menu – now those partially-transparent pixels around your black letters have been blended with red. You have Transparency Dithering options which are the same as for opaque images (see above).

The Interlaced option is like a Progressive Jpeg, the image will appear in a number of “scans”, at first appearing fuzzy then increasingly clear. This is used to reassure anyone reading your webpage that the image is actually turning up soon, but also increases file size.

Web snap determines the amount that colours are shifted toward web safe colours and prevents the browser reinterpreting your image colours. 100% web snap would make an image of only web safe colours.

Lossy reduces file size by loosing data. The more lossy your image is the smaller the file size and the rougher your image will look.

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