Shadow map depth range seems off

Sebastian Ahlman · 2016-02-18T17:23:48

I have been stumped by this for some hours now, so I figured I'd just ask here. I have had cascaded shadow mapping working for directional lights (ie. the sun) for some time now, and I just now implemented spot lights. For shadow mapping I use basically the exact same code as for the directional lights with the obvious exception that I use a perspective projection for the camera used when rendering the depth pass for the spot light. My problem is that the depth range seems off when rendering the spot lights. If visualizing the shadow map of the sun light and comparing to the shadow map of the spot light, I can only see the objects in the shadow map of the spot lights when extremely close to the objects (ie. when having the spot light extremely close to an object). For example, my near plane for the spot light camera is 0.1 and the far plane is set to the range of the light (eg. 100). Only when moving the light to within one unit from an object can you actually see any visual change in the shadow map visualization. In comparison, when rendering the directional lights, you can easily make out the objects on the shadow map even when high up in the sky, so to say. I do get correct shadows with the spotlight but only within a few world units away from the spot light which indeed suggest the depth values are all huddled together really close to the spotlight. Shadows further away from the light starts fading away, which I guess is because of the insufficient resolution of the depth map. Toying with the near/far distances helps a little, but not nearly enough. I know it's probably difficult for you to say where the problem is, but what would you look at next? Can I somehow scale the depth values into a more suitable range, say between 0.1 and 100 units away from the light source?

Graphics and GPU Programming Programming

Started by GuyWithBeard February 17, 2016 08:14 PM

9 comments, last by MJP 8 years, 2 months ago

MJP

20,295

February 18, 2016 05:23 PM

You don't want to reverse the viewport min and max depth, you can leave those alone. Those values are a clamp applied to your z/w value after rasterization, so you still want to leave them as 0 and 1 respectively.

You also need to make sure that you flip your depth test comparison function when rendering to your shadow map (use GREATER instead of LESS).

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