Why index colour?

[Risca]

Been getting some files lately with images in index colour mode? Does anyone else get these? Why would anyone use index. Shouldn't images be CMYK or Greyscale or maybe duotone/monotone?

[cari]

there's GOOD and BAD index.

Indexing could be merely mapping the colour information in an image into a chart. I always imagine that each pixel colour breakdown is analysed and mapped (in a table of some sort). It doesn't actually change the specific colour breakdown.

Bad index however is found in PNG-8 and GIF files, where 1000s, millions of colours are remapped to a maximum of 256 selectable colours: resulting in loss of quality.

Indexing can occur in Duotones that have been saved as EPS in pagelayout programs, and for which the files are converted to PDFs (this is an example of where I've seen it occur, this I consider to be GOOD index).

[CitrusZ28]

This mode uses at most 256 colors. When converting to indexed color, Photoshop builds a color lookup table (CLUT), which stores and indexes the colors in the image. If a color in the original image does not appear in the table, the?program chooses the closest one or simulates the color using available colors. Personally, I would stay away from insdexed color, unless it is used in a file that you created for special output. Most of thew time when customers send a file with indexed colors, it is a mistake. Convert to CMYK for 4-color output.

[Lammy]

I find most indexed color files come from web sites that someone is trying to turn into a printed piece. If it's a color piece i usually convert them to CMYK, and inform the CSR that it's going to look bad. If it's a two color job, I don't touch it and inform the CSR that it's not right and either the client needs to fix it or we need to charge for it.