Lost in Color Space

Color space conversion is annoying the hell out of me. Take a look at this picture, taken from the first Portland post. This was taken in Adobe RGB color space, but like all web images, was improperly converted to sRGB (Standard RGB) color space during upload.

This is the same image, except using Adobe's Adobe RGB to sRGB converter, instead of Google's direct translation. Notice the more vivid reds.

Shorter Scott: the colors are different and i'm not happy. A technical discussion as to why they are different follows, and if you process images with Photoshop it may behoove you to read on. Monitors typically define color as a function of three channels, or base colors: red, green, and blue. By varying the "colorfulness" (difference from gray) of each channel, you can simultaneously vary the hue, saturation, and brightness of your particular color. If you quantify the channel variations, the colors can actually be mapped to Cartesian (x, y, z) coordinates--they can be represented as a volume. When calibrated to a known color reference point, this color map becomes absolute--a true color space.

Each channel has 256 layers, so 256 x 256 x 256 = 16.7 million possible colors. Bitchin!

sRGB is by far the most common color space, mainly because our color photoreceptors (cone cells) are most sensitive to those three wavelengths. However, another tetrachromatic (4-channel) color space--CMYK--is preferred for print. These colors--Cyan, Maroon, Yellow, and Key black--correspond to ink used in most printers. CMYK color space encompasses a different area of color space, or gamut, than sRGB. This can result in inaccuracies while printing, and also means some colors possible in CMYK space cannot be represented in sRGB space.

Hence Adobe RGB--a color space designed to represent CMYK space using RGB colors. Speaking objectively, Adobe RGB represents an improvement upon sRGB because it can accurately portray a greater gamut than sRGB. Personally, i prefer Adobe RGB because it gives more vibrant reds and greens, which are important when shooting landscapes. It's also the default setting in Photoshop, so i was using Adobe RGB long before i knew there was a compatibility issue. A big one.

Here's the issue: both Adobe RGB and sRGB have 16.7 million possible colors, but Adobe RGB has a larger gamut. Therefore, it must represent a greater range of color using the same amount of colors. This means that the spacing between colors in Adobe RGB is greater than the spacing between colors in sRGB. Therefore, one color represented in Adobe RGB will have different color values of R, G, and B than the equivalent color in sRGB. Because of this mismatch, a red-155 in sRGB will be less vibrant than a red-155 in Adobe RGB.

With me so far? A particular RGB value in Adobe RGB is not the same color as the same value in sRGB. The sRGB color is less duller than the Adobe RGB color with the same value.

And now here's the problem: sRGB is the web standard--every photo site that i'm aware of, from Flickr to Facebook to deviantArt to Picasa, uses sRGB. When images are uploaded, the host will interpret Adobe RGB color values as though they were sRGB colors. This means that colors are changed during the upload process. The result? Images processed in Adobe RGB appear bland and flat, compared to sRGB images--even though Adobe should show a better range of colors.

Furthermore, because web browsers will automatically do this conversion, you simply cannot use Adobe RGB images for the web, without risking the loss of color information. Within Photoshop there is an option to accurately convert an Adobe RGB image into its sRGB equivalent, but this means all of my images to date have come out different than i've viewed them on my hard drive. Really this is just me being anal about my images, but really i just have this thing about when i'm not portrayed accurately [PDF].

You can read more about this problem here.