Sampling from a depth texture

[jwwalker]

I'm experimenting with a material whose fragment shader samples from a depth texture associated with a RenderTarget, and I'm wondering about a couple things. First, is this a legitimate thing to do? I'm new to GLSL_ES, and the language specification says some things that I don't really understand but sound scary, such as

If a non-shadow texture call is made to a sampler that represents a depth texture with depth comparisons turned on, then results are undefined.

Second, if the concept is OK, I was expecting the far plane to have depth 1, but experimentally it appears to have depth 0. Why?

[romainguy]

You can sample from a depth texture you rendered into, that's how shadow maps, ambient occlusion, etc. work internally.

The part of the spec you are quoting means that if you use texture() to sample from a depth texture, you'll get an undefined result if said texture has a depth comparison mode set (see TextureSampler::SamplerCompareMode). It should be fine unless you've set a compare mode on your texture sampler. If you have set a compare mode, you should use a shadow sampler and provide a Z value so the texture call returns 0 or 1 as the result of the comparison.

Where do you get a depth of 1? Note that we use reverse Z when we can to improve depth buffer precision.

[jwwalker]

When I said that I expected the far plane to have depth 1, I meant going by previous OpenGL experience (though it's certainly possible that I'm just confused). If you sometimes use reverse Z and sometimes not, how do I know how to interpret values obtained by sampling a depth texture?