sampler2DShadow bias component not working in GLSL?

Question

I am trying to implement shadow maps in opengl core 3.3. When I send a bias to the texture function in GLSL it doesn't seem to do anything at all. Am I using it wrong?

#version 330
uniform sampler2D diffusetex;
uniform sampler2D normaltex;
uniform sampler2D postex;
uniform sampler2D depthtex;

uniform sampler2DShadow shadowmap;
uniform mat4 shadowmat;

in vec2 uv;

layout (location=0) out vec4 outColor;


void main(){
    vec3 normal = normalize(texture(normaltex, uv).xyz);
    vec3 world_position = texture(postex, uv).xyz;

    vec4 shadowcoord = shadowmat*vec4(world_position, 1);
    float shadow = texture(shadowmap, shadowcoord.xyz,0.5);

    float luma = max(0.1,shadow);//ambient light
    outColor = texture(diffusetex, uv)*luma;
}

I am using linux, the nvidia proprietary drivers and golang. Doubt it has anything to do with it but just in case, there it is.

Answer 1

If you are trying to avoid shadow acne, you need to add that offset to your actual texture coordinates. The optional [bias] parameter in texture lookup functions is for LOD level computation, it is there to "sharpen" (negative value) or "soften" (positive value) mipmap filtering.

In this case, this would correctly apply a bias of 0.5:

vec4 shadowcoord = shadowmat*vec4(world_position, 1);
shadowcoord.z += 0.5;

Keep in mind that in window-space, 0.5 is ¹/₂ of your depth range, and probably not a good bias. Typically a good value would be closer to 0.001; 0.5 will probably kill all of your shadows.