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  1. I am a beginner in LAB attempting to apply the writings of Dan Margulis, who uses only Photoshop, to Affinity Photo. The LAB underlying functionality is mostly OK, but I have found that the AP user interface could be improved in several places. More experienced users, please comment. 1) In the Curves adjustment layer with LAB selected, the A channel curve is colored green and the B curve magenta. The appropriate colors are green left and magenta right for A; blue left and yellow right for B (with light or high luminosity on the right). 2) Creating a check box on the A and B curve windows for Intersect Midpoint would make it easier to construct color-intensifying curves. This is probably the most important basic move in LAB, and even Photoshop does not have this feature. 3) The Input Minimum and Maximum default to 0 and 1. Though I have not found a use for these, shouldn’t they be -1 to 1 or -100 to 100 or -127 to 127? 4) The Curves/LAB window’s second drop-down menu offers Master, L, A, B, and alpha. Is there a known use for ‘Master’ as in RGB? Nothing I have read about LAB indicates that there is. For that matter, is there any use for Master in CMYK? And how about ‘All’ as an alternative to Master in RGB? Furthermore, the histogram curves displayed for the LAB components here are not positioned the same as when one transforms the document to LAB and displays its histograms. May relate to previous point (3). 5) Photoshop can display the graphs of the various color space curves with the light (high-luminosity) side on the right or left (one-click toggle). This option would be useful to people learning from Margulis’s books, which show dark on the right. 6) Other Affinity users have called for improvements in the Curves window in general, not just for LAB: more precision and ways of placing nodes, a larger or scalable window, saving curves. Because of the sensitivity of the A and B curves to small movements, a fine control on node dragging would be handy, e.g., option-drag would reduce the motion by 1/5 or 1/10. Perhaps it already exists. 7) A LAB curves user on this forum called for a keyboard shortcut to move among the channels. Why not? He needs efficiency in a much-used workflow application. In the same vein, having selected the color space, instead of a second drop-down, radio buttons for the three or four channels would slightly simplify choosing one. 8) In the Channel Mixer adjustment layer, the LAB window A and B sliders need appropriate coloring like the RGB and CMYK. Just for consistency. 9) The Levels adjustment layer makes sense in RGB, but I am not so sure about LAB and CMYK. Does this lead to a useful correction? (Please help me, advanced users.) If you push the existing RGB colors to the AB limits of +/-127 won’t you just create out-of-gamut colors that will have to be distorted back in RGB space? And is gamma defined for this color space? Also, in the LAB window, shouldn’t the correct labels for the sliders be, for the A channel, green and magenta, and for the B, blue and yellow. Similar questions for CMYK. 10) The Apply Image filter and Channel Mixer adjustment layer seem as if they should have similar functionality, but their capabilities and interfaces are rather different. My understanding of Apply Image in Photoshop from Margulis is to blend or replace channels one at a time, sometimes across color spaces, but to do that in AP one has to create a gray scale image of a channel in a layer and make that the source. The destination seems to be all channels of a selected layer, or does one have to separate out those channels as well? The Channel Mixer is designed to specify a single channel as destination, but it appears the source must be in the same color space. Shouldn’t these two processes allow individual choice of source and destination channels in all color spaces? Are equations (Apply Image) and sliders (Channel Mixer) of equal value in practical applications? I don’t think this exhausts these issues, but this post has already exhausted me. My compliments if you have read this far. If you want to know more about Margulis, use a search engine or post here and I will try to summarize what I have earned from a large corpus of writing.
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