# Shadow mapping - Can't get the right texture coordinates

I'm trying to implement shadow maps for Spotlight's, but alas I can't get them to work. I have verified that my fragment shader is getting the shadow map texture. I can sample from it in the traditional sense and display it to the screen, so I know its there.

## My problem

• Actually getting the right coordinates and comparing that sample against the current depth value (No shadows appear)

If I'm understanding the process correctly, I have to transform the fragment from the space relative to the camera into the space occupied by the light when the shadow map was built.

I'm working with a deferred shading system, so I can easily get the current fragment's position in space by just sampling from a Position Map texture.

Here's how I'm trying to determine if a pixel is in shadow or not:

in vec3 EyePos;
in mat4 LightMVP;
out vec4 fragColor;

uniform SpotLight spotLight;
uniform vec2 ScreenSize;

uniform sampler2D PositionMap;
uniform sampler2D ColorMap;
uniform sampler2D NormalMap;
uniform sampler2D SpecularMap;

// Takes in transformed position, samples shadow map, outputs shadow factor float
{
vec3 ProjCoords = LightSpacePos.xyz / LightSpacePos.w;
vec2 UVCoords;
UVCoords.x = 0.5 * ProjCoords.x + 0.5;
UVCoords.y = 0.5 * ProjCoords.y + 0.5;
float z = 0.5 * ProjCoords.z + 0.5;

if (Depth < z + 0.00001)
return 0.0;
else
return 1.0;
}

// Returns the texture coordinate
vec2 CalcTexCoord()
{
return gl_FragCoord.xy / ScreenSize;
}

void main(void)
{
vec2 TexCoord = CalcTexCoord();                   // Used for sampling from input texture maps

vec4 WorldPos = texture2D(PositionMap, TexCoord); // Position in space

// ... omitted texture sampling for diffuse, normals, specular, etc ... //

vec4 LightSpace = (LightMVP) * WorldPos4;

fragColor = vec4(Color, 1.0) * CalcSpotLight(WorldPos.xyz, Normal, Specular, ShadowFactor);
}


The matrices lightV and lightP are the view and projection matrices used by the light when it takes a shadow map pass of the scene from its perspective.

#version 330

in vec4 vertex;

uniform mat4 mMatrix;
uniform mat4 pMatrix;
uniform mat4 vMatrix;
uniform mat4 lightV;
uniform mat4 lightP;
uniform vec3 eye;

out vec3 EyePos;
out mat4 LightMVP;

void main()
{
EyePos = eye;
mat4 worldMVP = pMatrix * vMatrix * mMatrix;
LightMVP = lightP * lightV * mMatrix;
gl_Position = worldMVP * vertex;
}


The rendered scene

## Questions

What am I doing wrong? Does any of this math seem off?

Does it matter if the shadow map texture is of a different resolution and aspect ratio than the camera? In this case it is exactly the same.

If nothing seems wrong, how else would I debug this then? I've tried just outputting a vec4 with the shadow factor, but all I get is bright white (as it isn't sampling right)

If it helps, I've been following the tutorials on OGLDev, so my deferred shading system is similar to theirs, and my shadow map is also like theirs (however they never did a tutorial for using both at the same time)

## Update

I managed to brute-force a solution, however I don't understand why it works, and I'm not certain if it is even the correct solution.

For calculating my LightMVP mat4, I mutliply the camera's projection matrix by the inverse light view matrix, which then gets passed into my fragment shader and multiplied by the world position, where the world position is derived from the texture Position Map.

LightMVP = (pMatrix) * inverse(lightV);
vec4 LightSpace = (LightMVP) * WorldPos;


Is there anything wrong with this approach?

• I updated the shaders in my post. For simplicity sake I put all my matrix operations in the fragment shader, however nothing changes when I multiply the light matrices together and output it to the fragment shader. – Yattabyte Sep 23 '15 at 21:59
• May you show the scene with and without shadows? The problem might come from another part than this shadow map. – Aracthor Sep 24 '15 at 1:25
• That is the scene without shadows. Any darkness is just the standard spot light mechanism for not filling in light outside of its cone. If I set the shadow factor to 0, nothing renders (as expected, everything becomes shadow), and if set to 1 then it looks the same as it does now. – Yattabyte Sep 24 '15 at 3:05
• the scene is hard to see with only your spot lights... Could you add a small ambient light to see every object? – Aracthor Sep 24 '15 at 4:31