Hi. To try to learn GIMP development, I have been looking into old bug #588681 (Airbrush tool produces banding with with soft brush). From the initial problem summary: "The Airbrush produces bandings when its used with a soft brush and low pressure. The sharp edges in the bands is [sic] very visible when it occurs in a otherwise soft image. The low pressure is very useful when I want to have good control over the spray." He uploaded an image as well of the effect, which I can easily reproduce. When using the airbrush, the severity of the effect is indirectly proportional to the pressure level selected. The exact same problem can be reproduced with a normal fuzzy brush, simply by selecting a low opacity, and repeatedly clicking on the canvas in the same location. Initially I spent a lot of time looking at app/core/gimpbrushgenerated.c, and especially gauss(), gimp_brush_generated_calc_lut(), and gimp_brush_generated_calc(). This is because I mistakenly supposed that the problem lay in the construction of the lookup table used initially to create the brush mask. However I discovered that this is completely unaffected by the brush opacity or airbrush pressure. Though having a far from sufficient understanding of how the GIMP brush painting process works, it seems to me like this is the fundamental problem: In making black-and-white brushes semi-transparent, GIMP somehow "levels out" the grey-scale, so that certain ranges of darkness become the same. On a fuzzy brush, the result is that the fine graduation is changed to appear stepped or banded. I would appreciate any insight into this matter. I have started to look into how brushes are made (applied?) semi-transparent in the code, but there are quite a few functions in the paint-funcs directory to look through. -- Christopher Howard http://indicium.us http://theologia.indicium.us
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