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.
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.
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.
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.
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?
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.