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Saturation layer blend mode

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Saturation layer blend mode Tom Hall 23 Sep 00:36
  Saturation layer blend mode yahvuu 27 Oct 14:52
   Saturation layer blend mode Nicolas Robidoux 27 Oct 19:32
    Saturation layer blend mode yahvuu 28 Oct 07:18
   Saturation layer blend mode Jim Michaels 28 Oct 22:17
Tom Hall
2011-09-23 00:36:49 UTC (about 13 years ago)

Saturation layer blend mode

Colours with a saturation of zero (Greys) have no hue value (stored as RGB). When increasing the saturation of such colours, "saturation" layer blend mode assumes they have a hue of zero, i.e. red.

I'm assuming using a HSV or HSL colour profile might avoid this (by having a redundant hue value even when saturation is zero), but I haven't found any such profiles to test.

Note that increasing saturation using the hue-saturation tool does not give the same result: white remains white, grey remains grey.

I would regard the saturation layer blend mode's result as unexpected behaviour for the following reasons: - Making assumption of non-existent hue data. - Not useful to end user. Controlling the saturation of parts of an image via a saturation layer doesn't work if the image contains any white (or grey) pixels. For example, an image of a green apple with white highlights will end up as a green apple with red or pink highlights. (With discontinuity being introduced as very pale green suddenly reaches pure white)
- Behaviour different from tool with same name.

Regards, Tom

yahvuu
2011-10-27 14:52:04 UTC (about 13 years ago)

Saturation layer blend mode

[re-posting to new mailing list]

Hi Tom and all,

Am 23.09.2011 02:36, schrieb Tom Hall: > I would regard the saturation layer blend mode's result as unexpected

the good news first off: the upcoming GEGL implementation of the "color space" layer modes uses LAB color space which gives much more useful results, if at the price of consuming more processor power. You can test-drive the new behaviour using "View -> Use GEGL" on a development version of GIMP.

The current implementation, in contrast, is based on the HSV color model which has some rather unpleasant characteristics.

For example, take a saturation blend of two gray orthogonal gradients, created with the default "use dithering" setting: http://yahvuu.files.wordpress.com/2009/08/hsv-blend_screenshot1.png The resulting image[1] is rendered with a stripe of highly saturated pixels, even though all source pixel are gray.

Accordingly, decomposition into HSV or HSL exhibits similar artifacts: http://yahvuu.files.wordpress.com/2009/08/decomposed.png Decomposing a gray gradient[3] using Colors->Components->Decompose gives unexpected non-black pixels in the saturation result.

So what's behind all this? It's not a bug in GIMP; it's a flaw of HSV/HSL, which does not properly reflect human perception of color. For example, consider the following example of a RGB HSV correspondance (assuming 8-bit RGB with range 0-255):

RGB HSV
--- ---
R = 0 Hue: 120

Nicolas Robidoux
2011-10-27 19:32:05 UTC (about 13 years ago)

Saturation layer blend mode

Yahvuu:

Would you be kind to briefly explain how Lab allows one to increase saturation with better results?

(Link to documentation and/or source code may do.)

Nicolas Robidoux

yahvuu
2011-10-28 07:18:38 UTC (about 13 years ago)

Saturation layer blend mode

Hello Nicolas,

Am 27.10.2011 21:32, schrieb Nicolas Robidoux:

Would you be kind to briefly explain how Lab allows one to increase saturation with better results?

pardon me, it's CIE LCH that is actually in use.

best regards, yahvuu

Jim Michaels
2011-10-28 22:17:58 UTC (about 13 years ago)

Saturation layer blend mode

a few frequently-used color systems:

HSV/HSL/HSB as a color "volume" looks different from RGB which looks different from CMYK. HSV/HSL/HSB: 2 cones with bases glued together. range of top to bottom of cones is Brightless/Lightness/Value and ranges from black to white. center to horizontal outer edge is Saturation, angle around horizontal circle is Hue (typically based on the 3 primary colors Red, Blue, Yellow).

RGB: 3D cube. each axis represents a range from black to Red or Green or Blue.

CMYK: 4D cube. each axis represents a range from black to Cyan or Magenta or Yellow or Black.

as you can see, representing these 3 color systems well is a challenge.