Shadow Map Produces Incorrect Results

Question

I'm attempting to implement shadow mapping into my deferred rendering pipeline, but I'm running into a few issues actually generating the shadow map, then shadowing the pixels – pixels that I believe should be shadowed simply aren't.

I have a single directional light, which is the 'sun' in my engine. I have deferred rendering set up for lighting, which works properly thus far. I render the scene again into a depth-only FBO for the shadow map, using the following code to generate the view matrix:

glm::vec3 position = r->getCamera()->getCameraPosition(); // position of level camera
glm::vec3 lightDir = this->sun->getDirection(); // sun direction vector
glm::mat4 depthProjectionMatrix = glm::ortho<float>(-10,10,-10,10,-10,20); // ortho projection
glm::mat4 depthViewMatrix = glm::lookAt(position + (lightDir * 20.f / 2.f), -lightDir, glm::vec3(0,1,0));

glm::mat4 lightSpaceMatrix = depthProjectionMatrix * depthViewMatrix;

Then, in my lighting shader, I use the following code to determine whether a pixel is in shadow or not:

// lightSpaceMatrix is the same as above, FragWorldPos is world position of the texekl
vec4 FragPosLightSpace = lightSpaceMatrix * vec4(FragWorldPos, 1.0f);

// multiply non-ambient light values by ShadowCalculation(FragPosLightSpace)
// ... do more stuff ...

float ShadowCalculation(vec4 fragPosLightSpace) {
    // perform perspective divide
    vec3 projCoords = fragPosLightSpace.xyz / fragPosLightSpace.w;
    // vec3 projCoords = fragPosLightSpace.xyz;

    // Transform to [0,1] range
    projCoords = projCoords * 0.5 + 0.5;

    // Get closest depth value from light's perspective (using [0,1] range fragPosLight as coords)
    float closestDepth = texture(gSunShadowMap, projCoords.xy).r;

    // Get depth of current fragment from light's perspective
    float currentDepth = projCoords.z;

    // Check whether current frag pos is in shadow
    float bias = 0.005;
    float shadow = (currentDepth - bias) > closestDepth  ? 1.0 : 0.0;

    // Ensure that Z value is no larger than 1
    if(projCoords.z > 1.0) {
        shadow = 0.0;
    }

    return shadow;
}

However, that doesn't really get me what I'm after. Here's a screenshot of the output after shadowing, as well as the shadow map half-assedly converted to an image in Photoshop:

Render output

Shadow Map

Since the directional light is the only light in my shader, it seems that the shadow map is being rendered pretty close to correctly, since the perspective/direction roughly match. However, what I don't understand is why none of the teapots actually end up casting a shadow on the others.

I'd appreciate any pointers on what I might be doing wrong. I think that my issue lies either in the calculation of that light space matrix (I'm not sure how to properly calculate that, given a moving camera, such that the stuff that's in view will be updated,) or in the way I determine whether the texel the deferred renderer is shading is in shadow or not. (FWIW, I determine the world position from the depth buffer, but I've proven that this calculation is working correctly.)

Thanks for any help.

Answer 1

Debugging shadow problems can be tricky. Lets start with a few points:

1. If you look at your render closely, you will actually see a shadow on one of the pots in the top left corner.

2. Try rotating your sun, this usually helps to see if there are any problems with the light transform matrix. From your output, it seems the sun is very horizontal and might not cast shadows on this setup. (another angle might show more shadows)

3. It appears as though you are calculating the matrix correctly, but try shrinking your maximum depth in glm::ortho(-10,10,-10,10,-10,20) to tightly fit your scene. If the depth is too large, you will lose precision and shadow will have artifacts.

4. To visualize where the problem is coming from further, try outputing the result from your shadow map lookup from here:

closestDepth = texture(gSunShadowMap, projCoords.xy).r

If the shadow map is being projected correctly, then you know you have a problem in your depth comparisons. Hope this helps!