Benergizer Posted November 29, 2020 Share Posted November 29, 2020 I was just playing with some brushes just now and came across something that I'm not sure is a feature or a bug. When using a brush with a very low flow (2%, 0% hardness) the brush never reaches full opacity. it looks like it does at one point, but as I continue to paint the inside starts to become lighter. I provided a picture using 100% black agaisnt a 50% gray background to give an example. For the 2% flow brush I repeatedly moved the brush up and down (without lifting it) until the value not longer changed. I'm not sure it this is the way it's supposed to work, or if it's a bug. Everything else is set to 100% (Opacity, accumulation, shape) spacing set to 5% with wet edges set to "don't set wet edges." Quote Link to comment Share on other sites More sharing options...
lepr Posted November 29, 2020 Share Posted November 29, 2020 The apparent increase in lightness toward the centre is an optical illusion. You can measure the pixel values to prove that. The brush puts down a series of dabs of its tip and each dab has an opacity equivalent to the flow percentage. The opacity not reaching 100% is the result of the compositing rule. The mathematics results in each overlaid dab contributing less to the total opacity than the previous dab. If the calculations were performed with infinite precision, 100% opacity could never be achieved when flow is not 100%. When a dab's contribution to the total opacity of overlaid dabs is sufficiently small relative to the numerical format (for example, 8-bit integer or 16-bit integer) of the document, that contribution will be rounded to zero and so the total opacity of the brush stroke ceases to increase beyond a limit that is below 100%. dab1 + dab2 + dab3 + dab4 + ... flow 25%: 0.25 + (0.75*0.25) + (0.75*0.75*0.25) + (0.75*0.75*0.75*0.25) +... 0.25 + 0.188 + 0.141 + 0.105 +... flow 2%: 0.02 + (0.98*0.02) + (0.98*0.98*0.02) + (0.98*0.98*0.98*0.02) +... 0.02 + 0.0196 + 0.0192 + 0.0188 +... walt.farrell 1 Quote Link to comment Share on other sites More sharing options...
Benergizer Posted November 29, 2020 Author Share Posted November 29, 2020 That's very interesting. I always, assumes that with a low flow brush, the brush would eventually build itself up to 100% over time. But what you say makes sense. Each dab is only giving 2% flow, it's impossible for it to ever reach 100% because it's on;y ever giving a percentage of 100%. Kind of like halving a number, then continuing the half the result. You would never reach true 0%. I assume the same is for opacity then? if I have a brush set to 50%, it will never achieve true 100% opacity, because it's only ever putting down 50%. Make a lot of sense. Thank you. Quote Link to comment Share on other sites More sharing options...
lepr Posted November 29, 2020 Share Posted November 29, 2020 By the way, you should work in a 16 bpc document to avoid the distinct banding that tends to result from accumulation of Affinity's non-dithered brush tip in an 8 bpc document. Quote Link to comment Share on other sites More sharing options...
Benergizer Posted November 29, 2020 Author Share Posted November 29, 2020 Yes, I've definitely noticed that. I use the low flow with the clone stamp tool for portrait retouching, so the banding isn't an issue when doing skin. But it definitely becomes noticeable when I am removing hair fly-aways against a solid background. I've tried different ways to minimize it, but like you said changing to 16-bit it the only thing that makes it go away. Quote Link to comment Share on other sites More sharing options...
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