The first attached pic shows the screen before switching on Black-and-White.
The second shows after.
The result is quite sensitive to moving the red and green sliders, but moving the blue has no effect at all.
Also, I’ve set quite big adjustments on the red and green, expecting to see a big difference in the brightnesses of the relevant squares, but to my eye this hasn’t happened.
Am I misunderstanding what should be happening here?
Thanks,
Andrew
I think you may have the wrong B&W settings for the effect you want, in my test with screen pic shown, moving the blue slider changes the various color squares as expected.
Settings, Method=Channel Mixer, Presets=Relative RGB, Color Filter=None, move the Blue Slider under Presets not the Gamma settings .
Thanks David, that works and I can do what I wanted. However, I’m still finding the red and green gamma sliders do something, but not the blue. Is it the same for you? - if so I’ll log it as a bug this coming week. Andrew
For many times I get asked this question and the answer is always the same.
The main reason is the design of the space Prophoto. Indeed, particularly in the blue, it contains colors that are invisible to humans.
For low luminance values, Prophoto gamut is huge.
I enclose a graph gamut sRGB, Adobe and Prophoto.
Thus the linear matrix coefficients are very low for prophoto, which explains the lack of reaction.
This is absolutely not a bug Gamut
Another oddity I’ve noticed is about the same scenario i.e. Method=Channel Mixer, Presets=Relative RGB, Colour Filter=None. I increased the upper Red slider (not the gamma one) intending to lighten some bricks, but not much happened. In fact maximum Red emphasis seems to be at -67! There is a “singularity” at -66, a black hole in fact! - since the image goes completely black. Then at -65, I think you have minimum Red contribution. So perhaps there is a maths error somewhere? Also, the percentages displayed say “-1.$%” - yes, dollar sign.
This also happens with the green and blue sliders.
It also happens in Adobe RGB space, not just Prophoto.
I have to say I don’t understand the value range of -100 to +200. Would it be simpler to have 0 to say 100, with 0 meaning zero contribution from the channel, and 100 meaning the full amount present in the colour image?
Having worked with software myself, I point these things out knowing they are easy to say, but not necessarily easy to fix!
The mixer uses a range of -100 to 200 consider negative values and thereby increase the possibility of special effects.
There is currently a bug when the sum of the 3 channels is zero. This can cause display problems or a crash, but did not question the algorithm. I would make a fix in a future update
hi @jdc, thanks for the comments.
I hadn’t spotted the big thread on the “main” channel mixer when I posted above, perhaps this will help my understanding of the B&W mixer. Quite a read though! Andrew
@jdc Hi Jacques,
I wonder if, a working color space covering as much as possible of the CIE but with visible primaries like rec 2020 Rec. 2020 - Wikipedia would be a better compromise ?
