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Out of Gamut: Realizing Good Intentions with Rendering Intents
Color geek extraordinaire Bruce Fraser gets to the bottom of rendering intents, and when to use each to get the best results.
Written by Bruce Fraser on April 4, 2001
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Relative Colorimetric Rendering
The other three rendering intents all translate the white of the source to the white of the output, and shift all the other colors accordingly. Since we usually judge our output in the context of its native white, we almost always want to use one of these three rendering intents that perform white point conversion, unless we're proofing as described in the previous paragraph.
Relative colorimetric rendering is similar to absolute colorimetric rendering. The only difference is that relative colorimetric scales the white point of the source to the white point of the target. Like absolute colorimetric rendering, it clips out-of-gamut colors to the nearest reproducible hue.
Figure 3: Converted to an inkjet space using relative colorimetric rendering. Again, discontinuities are obvious, but the grays appear more natural.
Perceptual Rendering
Perceptual rendering attempts to compress the gamut of the source space into the gamut of the target space. Exactly how this is accomplished is left to the discretion of the tool used to build the profile, but typically perceptual rendering desaturates all colors to bring the out-of-gamut colors into the target gamut while more or less maintaining the overall relationship between colors. Preserving the relationship between colors helps preserve the overall appearance of images.
Figure 4: Converted to an inkjet space using perceptual rendering. Perceptual rendering preserves smooth gradients across the colors.
Saturation Rendering
The final rendering intent -- saturation rendering -- maps the saturated primary colors in the source space to the saturated primary colors in the target space, without bothering about differences in hue, saturation, or lightness. It's designed for rendering business graphics like pie and bar charts, where we simply want vivid colors and aren't particularly concerned as to exactly what those colors are.
Figure 5: Converted to an inkjet space using saturation rendering, which preserves smooth gradients but shifts the colors.
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Perceptual rendering
Michael,
It's hard to make definitive statements about perceptual rendering, because this is really where profiling tools differentiate themselves -- each one has it's own secret sauce.
Generally, there's some weighting -- it isn't just a linear desaturation -- and in-gamut colors typically get less desaturation than out-of-gamut ones.
The effect is usually quite subtle because our eyes tend to judge relative color, rather than absolute, and because if you're making the judgements on the monitor, what you're looking at is pretty close to sRGB anyway.
I'm trying to find out what happened to part II of sharpening -- it was here, but it seems to have dropped off the list on my author's page. I've sent the appropriate inquiries to teh appropriate powers...
is perceptual rendering as simple as that?
Your description of perceptual rendering, which is to essentially move all RGB values into gamut, might seem to imply more desaturation than we actually see. That is, we might expect extreme desaturation if we move from ProPhotoRGB to sRGB ... and less if from ProPhoto to AdobeRGB. While I watch for this, I seem to experience it only very subtly, if at all. I wonder if this might be because the perceptual rendering process is somehow weighted, or if some aspect of gamut is given special consideration. How does this work?
shAf
(P.S., isn't there a second article on sharpening which should be on your list of fine articles?)
question
is there a way to check in photoshop what rendering intent is used in an image with an embedded profile?