indexed color spaces

[mattbeals]

I don't know how much I would trust Wikipedia, but basically. That's why I equate indexed RGB or indexed CMYK as a lossless form of image compression. You're basically "indexing" the bits of data and creating a look up table to save space. It's my understanding that the PDF library itself, whether it be Distiller or not, decides to index images with less than 256 unique colors when generating the PDF.

Obviously for print production there would no reason to use indexed mode in PhotoShop.

[gordo]

Originally Posted by mattbeals

I don't know how much I would trust Wikipedia, but basically. That's why I equate indexed RGB or indexed CMYK as a lossless form of image compression. You're basically "indexing" the bits of data and creating a look up table to save space. It's my understanding that the PDF library itself, whether it be Distiller or not, decides to index images with less than 256 unique colors when generating the PDF.
Obviously for print production there would no reason to use indexed mode in PhotoShop.

The definition of indexed color in Wikipedia is the same as the definition you'll find in any graphic arts book - including the educational ones from vendors such as Heidelberg and Agfa. None that I have found (and I've got a ton of reference material) ever mentions conversion to indexed mode in PDF creation.
So, I have only experienced indexed color in PShop where it's primarily used to create GIFs or reduce image size for apps like Powerpoint. When I read the original post, (and I did say then that I may of misunderstood the question) I thought that he was referring to the original images being supplied in indexed format rather than a conversion being applied "under the hood" during PDF creation.

Hopefully "leonardr" from Adobe will clarify this issue a bit more.

thx Matt.

gordo

[leonardr]

Wikipedia and Matt are both correct -0 so let's separate things out and maybe it will help.

First - wikipedia is correct as far as the definition of how an indexed image (color space) is stored (regardless of file format - PDF, GIF, etc.). Instead of having color values in the actual image, you have color values in a "palette" (or lookup table) and then the image is just a set of indices into that palette/table. The only thing worth noting is that every other use for indexed color is RGB only - which is why many folks (as Gordo did) ASSUME that indexed == RGB. PDF (and Postscript!), however, support a MUCH MORE flexible definition of indexed, in that the indexing works on ANY (or almost any) of the 11 different color spaces of PDF. So you can have indexed RGB and indexed CMYK as well as indexed ICC, indexed Spot/Separation and indexed DeviceN.

The second part of the discussion is what Adobe's products do when creating and/or optimizing PDF documents. As Matt (and Michael) both said, when our products detect that use of an image that has <256 values - it will automatically be converted to the indexed form in order to reduce file size. There is no option for this because it is 100% lossless - the color VALUES aren't changed, they are just moved out of the image and into a palette. What you did in Photoshop, with >256 colors being reduced to 256 is called quantization - and we do NOT do that when converting.

Hopefully that helps clarify this issue.

[michaelejahn]

Originally Posted by gordo

So Adobe Distiller does the conversion to indexed color (automatically?)

jahn coments - yes - and so does the Adobe PDFLib (for example, when you place an image that contains less than 256 colors - which could be an Orange on an Orange background, or a red apple on a red background, for example) - the image will jump from RGB or CMYK to an index RGB or an Index CMYK image..

Originally Posted by gordo

if the image contains less than 256 colors while maintaining its CMYK or RGB association. Because the image uses a range of colors that fall within the (practically) 256 limit of indexed color mode, you can compress the image without introducing artifacts. Correct?

ABSOLUTELY - there, you have it exactly - and then, the issue becomes, that Adobe can't eat its own dogfood. That is, now - i can no longer use that Adobe Touch-up Object tool to open and edit an image in Adobe Photoshop.

In you example (the circular rainbow - gee, I never knew you were from San Fran!) - anyway, this image cannot be represented properly using only 256 distinct colors, so you will not get that image to automatically index using the Adobe PDF Lib - or Distiller.

Hope this helps !

[gordo]

Thanks all,

This auto-indexing of color by Adobe is something I hadn't read about anywhere and, since I'm not a big user of PDFs, I haven't come across it in my work.
So, thank you. I've learned something.

best, gordo

[michaelejahn]

@ Gordo,

Yes, indeed. It was the bane of my existence for a while. This also is an issue when people scan black and white documents in grayscale, then use the Adobe Acrobat PDF Optimizer tool (in Acrobat 7, under the Tool menu > Print Production > PDF Optimizer, in Acrobat 8 and 9, under documents menu > Optimize Scanned PDF) - you can no longer use the touch up object tools (depending on certain settings - like Deskew for example) - people can either delete on object, but the can't open and clone a spec of dirt off an image in photoshop, for example.

Other vendors like ELAN GMK and IoFlex provide deskew and other clean up tools that then create PDF files that can still be edited using the Acrobat touch up object tool > open in Photoshop method.

[gordo]

Thanks again Michael. And I do appreciate that you didn't use all CAPs in your responses to me.

best, gordo

[assinippi]

michaelejahn

Unfortunately your trick doesn't work on OS X 10.5.4.