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copied!<p>You may want to render a black rectangle with alpha going from 1.0 to 0.0 using glBlendFunc (GL_ONE, GL_SRC_ALPHA).</p> <p>Edit in response to your comment (reply doesn't fit in a comment):</p> <p>You cannot fade single pixels depending on their age with a simple fade-to-black operation. Usually a render target does not "remember" what has drawn to it in previous frames. I could think of a way to do this by alternatingly rendering to one of a pair of FBOs and using their alpha channel for it, but you needed a shader there. So what you would do is first render the FBO containing the pixels at their previous positions, decreasing their alpha value by one, dropping them when alpha == 0, otherwise darkening them whenever their alpha decreases, then render the pixels at their current positions with alpha == 255.</p> <p>If you only have moving pixels:</p> <pre><code>render FBO 2 to FBO 1, darkening each pixel in it by a scale (skip during first pass) render moving pixels to FBO 1 render FBO 1 to FBO 2 (FBO 2 is the "age" buffer) render FBO 2 to screen </code></pre> <p>If you want to modify some scene (i.e. have a scene and moving pixels in it):</p> <pre><code>set glBlendFunc (GL_ONE, GL_ZERO) render FBO 2 to FBO 1, reducing each alpha > 0.0 in it by a scale (skip during first pass) render moving pixels to FBO 1 render FBO 1 to FBO 2 (FBO 2 is the "age" buffer) render the scene to screen set glBlendFunc (GL_ONE, GL_SRC_ALPHA) render FBO 2 to screen </code></pre> <p>Actually the scale should be (float) / 255.0 / 255.0 to make the components equally fade away (and not one that started at a lower value become zero before the others do).</p> <p>If you only have a few moving pixels, you could re-render the pixel at all previous positions up to 255 "ticks" back.</p> <p>Since you need to re-render each of the pixels anyway, just render each one with the proper color gradient: Darker, the older the pixel is. If you have a real lot of pixels, the dual FBO approach might work.</p> <p>I am writing ticks, and not frames, because frames can take a varying amount of time depending on renderer and hardware, but you probably want to have the pixel trail fade away within a constant time. That means you need to dim each pixel only after so-and-so many milliseconds, keeping their color for the frames in between.</p>

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