Safely
Increase Saturation
How To:
Safely increase RGB image saturation without creating artifacts such as posterization.
The cropped and resized image shown below was originally made available by Andrew Rodney as a raw camera file to members of the Dan Margulis Applied Color Theory in Photoshop email list for colour space, image editing and wide gamut output evaluation purposes. This image is a rather extreme but not unique example of the problems encountered when increasing saturation in subtly detailed images and makes an excellent test image. Although these issues can be encountered in different colours, this appears to be a particular problem for images containing important yellow hues (Andrew provided three different shots of yellow flowers that exhibited similar editing issues).
Image Editing & Working Space Evaluation
Before I make any observations, I encourage you to use the radio buttons below to compare the various colour modes, colour spaces and image editing techniques (all images have been converted to sRGB for internet display). In all cases, a +15 point increase in saturation was performed using Photoshop's Hue/Saturation command.
Notes:
Click on each radio button above and wait for the image to download (images should then be cached in your browser to speed further viewing).
1. Compare the three images noted above in bold text. Although there are differences in saturation, the three images closely resemble each other in tonality and detail. One would expect that Photoshop's Hue/Saturation command would only effect the saturation of an image when the saturation slider is adjusted in RGB mode, however this is not the default behaviour when making adjustments with this slider. A Saturation or Color blend must be performed so as not to affect the images luminosity.
2.
When using Normal blend mode in ProPhoto RGB, luminosity is altered with the increase of saturation. Switching to Color or Saturation blending modes addresses this issue leaving the original luminosity unaffected.
3.
Normal blend mode in sRGB produces posterization in the shading in the centre right yellow petals (medium sized spaces such as Adobe RGB have similar issues). Using Color or Saturation blending modes reduces the posterization, as does working in a wide gamut space (either Lab or RGB).
Further Information:
Lab mode saturation slider edits do not affect the tone of the image, posterization is also avoided (due to the wide gamut of Lab and separation of tone from colour in the underlying channel structure). Conversions to Lab mode may not be considered ideal, due to the visually lossless errors introduced by Lab mode.
ProPhoto RGB provides similar results to Lab mode edits when blended in Color mode. In Normal blending mode, luminosity is affected despite saturation being the only colour control altered. Unlike medium gamut RGB spaces such as Adobe RGB or smaller spaces such as sRGB, the luminosity alteration does not adversely affect the detail in this particular image.
When editing saturation in working spaces that are smaller than ProPhoto RGB or Wide Gamut RGB, it is recommended that one compare the saturation edit using Color or Saturation blending modes. If boosting saturation causes posterization in Color or Saturation blend modes, one can make a temporary trip to a wider gamut space such as ProPhoto RGB or Lab mode, which offer more room for large saturation edits (16bpc editing is strongly recommended for wide gamut colour space editing).
Using Color/Saturation blends and wide gamut editing spaces only address part of the issue. The Hue/Saturation command is an old tool lacking fine control, it globally increases saturation at all saturation levels, when higher saturation should ideally be ignored. With this in mind, a Saturation Mask can be used to target pixels of lower saturation while ignoring more saturated pixels. Pixmantec RawShooter Pro, Adobe Lightroom, Adobe Camera Raw and Adobe Photoshop CS4 feature the Vibrance command, which overcomes many of the issues mentioned with simple default Saturation slider adjustments in RGB mode.
Further Reading:
The Lights Right - Saturation Mask Actions