OpenGL: ShadowMapping: Shadows only visible at y=0

Question

hello, i have implemented shadowmapping with an FBO and GLSL. it is used on a heightfield. that is some objects (trees, plants, ...) cast shadows on the heightfield.

the problem i have, is that the shadows are only visible on the ground of the heightfield. that is, where the heightfield's height = 0. as soon as there is some height involved, the shadows disappear. if i look at the shadowmap itself, everything looks fine... objects that are closer to the light are darker.

here is my GLSL vertexshader:

uniform mat4 lightView, lightProjection;

const mat4 biasMatrix = mat4(   0.5, 0.0, 0.0, 0.0,
       0.0, 0.5, 0.0, 0.0,
       0.0, 0.0, 0.5, 0.0,
       0.5, 0.5, 0.5, 1.0);     //bias from [-1, 1] to [0, 1]

void main()
{   
gl_Position = ftransform(); 

mat4 shadowMatrix = biasMatrix * lightProjection * lightView;
shadowTexCoord = shadowMatrix * gl_Vertex;    
}

fragmentshader:

uniform sampler2DShadow shadowmap;
varying vec4 shadowTexCoord;

void main() 
{    
vec4 shadow = shadow2DProj(shadowmap, shadowTexCoord, 0.0);
float colorshadow = shadow.r < 0.1 ? 0.5 : 1.0;

vec4 color = vec4(1,1,1,1); 

gl_FragColor = vec4( color*colorshadow, color.w );

}

thanks a lot for any help on this!

Answer 1

Maybe you just missed it copy-pasting, but I don't see shadowTexCoord as varying in the vertex shader. This should result in a compilation error, though.

Answer 2

I think there might be some confusion between the different spaces here. As written, it looks like your code would only work if gl_ModelViewMatrix for the ground contains only camera transformations. This is because ftransform basically goes

gl_Position = gl_ProjectionMatrix * (gl_ModelViewMatrix * gl_Vertex)

that means that the gl_Vertex is specified in object coordinates. However typically the view matrix of the light maps from world coordinates to the light's view space so this code would only work if object space = world space. So basically, lets say you scale the terrain, well then object space doesn't equal world space anymore. Because of this you need to separate out the gl_ModelViewMatrix into two parts: the camera view matrix and the modeling transform (eg object -> world space)

I havent tested this code, but I would try something like this:

uniform mat4 lightView, lightProjection;
uniform mat4 camView, camProj, modelTrans;
const mat4 biasMatrix = mat4(   0.5, 0.0, 0.0, 0.0,
                                                        0.0, 0.5, 0.0, 0.0,
                                                        0.0, 0.0, 0.5, 0.0,
                                                        0.5, 0.5, 0.5, 1.0);        //bias from [-1, 1] to [0, 1]

void main()
{
mat4 modelViewProjMatrix =  camProj * camView * modelTrans;  
gl_Position =  modelViewProjMatrix * gl_Vertex;   

mat4 shadowMatrix = biasMatrix * lightProjection * lightView * modelTrans;
shadowTexCoord = shadowMatrix * gl_Vertex;    
}

Technically it's faster to multiply the matrices on the CPU and only pass the exact ones you need but for getting stuff working sometimes its easier to do this way.