[vanderZwan]

That works if more overdraw = more intensity is all you care about, and may very well be good enough for many kinds of charts. But with heat map plots one usually wants a proper mapping of some intensity domain to a color map and a legend with a color gradient that tells you which color represents which value. Which requires binning, counting per bin, and determining the min and max values.

[akomtu]

Emm.. no, you just do one render pass to a temp framebuffer with 1 red channel, then another fragment shader maps it to an RGB palette.

[vanderZwan]

Wait, does additional blending let you draw to temp framebuffers with high precision and without clamping? Even so you'd still need to know the maximum value of the temp framebuffer though.

[akomtu]

That's what EXT_float_blend does. It's true, though, that you can't find the global min/max in webgl2. This could be done, theoretically, with mipmaps if only those mipmaps supported the max function.

[vanderZwan]

Couldn't you do that manually with a simple downscaling filter? I'd be very shocked if fragment shaders did not have a min or max function.

Repeatedly shrinking by a factor of two means log2(max(width, height)) passes, each pass is a quarter of the pixels of the previous pass so that's a total of 4/3 times the pixels of the original image. Should be low enough overhead, right?

[akomtu]

Sure, that will work, but it's log2 passes + temp framebuffers. As for overhead, I'm afraid it will eat a couple fps if you run it on every frame. In practice, though, I'm not sure that finding the exact maximum is that valuable for rendering: a good guess based on the dataset type will do. For example, if you need to render N points that tend to congregate in the center, using sqrt(N) as the heuristic for the maximum works very well.