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I have a TIFF image and a JPG image. http://regex.info/exif.cgi says that they both use a CMYK color space labelled as "SWOP (Coated), 20%". How do I accurately and losslessly convert the TIFF image into an sRGB PNG and the JPG image into an sRGB JPG?

Hiccup
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It's unlikely you will be able to do this losslessly if for no other reason than CMYK covers a different (but overlapping) color space than sRGB. Additionally, there are several algorithms for converting between the two color spaces, each of which produces slightly different results.

From Understanding Color:

Graphs of the color spaces for CMYK, sRGB, and AppleRGB

Converting the TIFF to PNG along with the color space may be done relatively losslessly. As you can see from the graph, blue-greens and bright yellows may be a problem when converting from CMYK to sRGB.

Converting the color space on a JPG will be lossy since changing the colors necessarily changes the compression due to the nature of JPEG compression.

Adobe has these instructions for doing color space changes in Photoshop:

  1. Choose Edit > Convert To Profile.
  2. Under Destination Space, choose the color profile to which you want to convert the document’s colors. The document will be converted to and tagged with this new profile.
  3. Under Conversion Options, specify a color management engine, a rendering intent, and black point and dither options (if available). (See Color conversion options.)
  4. To flatten all layers of the document onto a single layer upon conversion, select Flatten Image.
  5. To preview the effects of the conversion in the document, select Preview.

Adobe Illustrator, InDesign, and Lightroom can also do color space conversions.

Ouroborus
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  • Is there a modern, well-supported color space that includes all the colors CMYK does? To convert to instead of converting to sRGB. Or is there really no need, as the images will look the same if you aren't zooming in on the pixels? Also, is there a free program that will do the same job? – Hiccup Oct 20 '16 at 18:10
  • @Hiccup Unfortunately, no. CMYK is meant for print and surrounds the use of reflection (colored pigments) to display color while RGB is meant for television and the like and surrounds the use of emission (colored filters, etc.) to display color. The narrower color space of CMYK was well established before RGB became available. Pantone is a popular color scheme for print that allows colors that can't be reproduced in CMYK but Pantone has other constraints. Generally, photography is done in a "widened" RGB and then graphic artists do what's necessary to make color corrections for print. – Ouroborus Oct 20 '16 at 18:25
  • Two more questions: 1. can you convert a file to a different color space losslessly - other than the loss of color? I.e. no extra jpeg artefacts? 2. is this system of two "incomplete" color spaces the best scenario - wouldn't it work better if there was one which covered all visible colors? – Hiccup Oct 21 '16 at 18:26
  • @Hiccup For JPG, no. The only thing you can do losslessly to JPG is cropping, rotating, and re-slicing and those are only possible if their boundaries match up with the internal chunking that JPEG does. (There's purpose-built software for doing this; things like Photoshop usually don't bother.) Basically, operations that can be applied at the chunk level. Once you start messing with the contents of a chunk, the chunk would need to be subjected again to the lossy compression. You may want to experiment. If you keep the compression level the same, the impact should be minimal. – Ouroborus Oct 21 '16 at 19:36
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    @Hiccup You've seen the chart. You can see that 90%+ of the CMYK color space falls within the RGB color space. I imagine the bit of loss is considered acceptable. In my day job, I've noticed that high quality images are typically provided as CMYK TIFFs. I don't really notice any deterioration when converting to RGB. A side-by-side comparison does show a slight difference, but the public eye doesn't get that privilege. – Ouroborus Oct 21 '16 at 19:44
  • Thanks for the explanation. I've marked your answer as accepted. I've found Photoshop to be quite useful with handling different color spaces with JPEGs, PNGs, TIFFs and PSDs. – Hiccup Oct 22 '16 at 17:14