Return-path: <scott@asofyet.org> > I can manipulate the "fill_color_rgba" hex number to > give me a few different semi-transparent ellipses, but > can anyone explain how the hex numbering actually works to > produce the semi-transparent effect? The RGBA numbers are 32-bit values composed of one-byte-per-channel for red, green, blue, and alpha (0xrrggbbaa). The alpha value is the "opacity", and this is used to describe how much of the source color is blended with the destination color to create the output color according to this formula: alpha_pct = alpha / 255.0 destination red' = source red * alpha_pct + dest red * (1.0 - alpha_pct) destination green' = source green * alpha_pct + dest green * (1.0 - alpha_pct) destination blue' = source blue * alpha_pct + dest blue * (1.0 - alpha_pct) An alpha value of 0xff is 255, which is 100% of the source color, which is a traditional "source-copy" operation. An alpha value of 25% means the output color is composed of 25% of the source color and 75% of the color already existing at the destination. The actual color you'll get as a result of the opacity value, therefore, depends on what you're blending it with. On a light background, a lower opacity makes the color lighter, on a dark background, a lower opacity makes the color darker, over red it gets redder, etc. This is implemented using client-side blitting of microtiles in libart. To get proper alpha blending, you need full-resolution color data, so it must be done on 24-bit RGB values -- dithered colors don't work --- so it happens on the client side, using XPutImage to transfer the full RGB to the X server. > The canvas demo, is set up, to allow you to choose a normal > (not anti-aliased) example, which does it on the basis of using a > stiple. > It's alot uglier, but it is always how I thought it was done, > coming from > Tk. Stippling is a form of halftoning, which is how you approximate colors on a device that can't actually reproduce them. By having a stipple pattern that displays every other dot, you get effectively 50% of the input color, and the background shows through. No alpha blending occurs --- it's the equivalent of alpha=100%. And, yes, this is uglier than alpha blending, but tends to be much faster, as it can be performed on the X server. The Tk canvas pretty much predates widespread use of alpha blending; old hardware had a hard time with it, because it requires keeping full-color copies of the various components to be blended. As an aside, cairo does everything in an alpha space, and is much more efficient at it than libart. Part of the big fuss about the new compositing-based X servers is that they can do alpha blending on the server side.
In reply to Re^2: Drawing venn diagrams
by zentara
in thread Drawing venn diagrams
by sandrider
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