Webgl: alternative to writing to gl_FragDepth

Question

In WebGL, is it possible to write to the fragment's depth value or control the fragment's depth value in some other way?

As far as I could find, gl_FragDepth is not present in webgl 1.x, but I am wondering if there is any other way (extensions, browser specific support, etc) to do it.

What I want to archive is to have a ray traced object play along with other elements drawn using the usual model, view, projection.

Answer 1

There is the extension EXT_frag_depth

Because it's an extension it might not be available everywhere so you need to check it exists.

var isFragDepthAvailable = gl.getExtension("EXT_frag_depth");

If isFragDepthAvailable is not falsey then you can enable it in your shaders with

#extension GL_EXT_frag_depth : enable

Otherwise you can manipulate gl_Position.z in your vertex shader though I suspect that's not really a viable solution for most needs.

Answer 2

Brad Larson has a clever workaround for this that he uses in Molecules ( full blog post ):

To work around this, I implemented my own custom depth buffer using a frame buffer object that was bound to a texture the size of the screen. For each frame, I first do a rendering pass where the only value that is output is a color value corresponding to the depth at that point. In order to handle multiple overlapping objects that might write to the same fragment, I enable color blending and use the GL_MIN_EXT blending equation. This means that the color components used for that fragment (R, G, and B) are the minimum of all the components that objects have tried to write to that fragment (in my coordinate system, a depth of 0.0 is near the viewer, and 1.0 is far away). In order to increase the precision of depth values written to this texture, I encode depth to color in such a way that as depth values increase, red fills up first, then green, and finally blue. This gives me 768 depth levels, which works reasonably well.

EDIT: Just realized WebGL doesn't support min blending, so not very useful. Sorry.