In my previous question, I asked whether it is possible to do projective texturing with deferred lighting. Now (more than half a year later) I have a problem with my implementation of the same thing. I am trying to apply this technique in light pass. (my projector doesn't affect albedo). I have this projector View a Projection matrix:

Matrix projection = Matrix.CreateOrthographicOffCenter(-halfWidth * Scale, halfWidth * Scale, -halfHeight * Scale, halfHeight * Scale, 1, 100000);
Matrix view       = Matrix.CreateLookAt(Position, Target, Vector3.Up);

Where halfWidth and halfHeight is are half of the texture's width and height, Position is the Projector's position and target is the projector's target. This seems to be ok. I am drawing full screen quad with this shader:

float4x4 InvViewProjection;

texture2D DepthTexture;
texture2D NormalTexture;
texture2D ProjectorTexture;

float4x4 ProjectorViewProjection;

sampler2D depthSampler = sampler_state {
    texture = <DepthTexture>;
    minfilter = point;
    magfilter = point;
    mipfilter = point;

sampler2D normalSampler = sampler_state {
    texture = <NormalTexture>;
    minfilter = point;
    magfilter = point;
    mipfilter = point;

sampler2D projectorSampler = sampler_state {
    texture = <ProjectorTexture>;
    AddressU  = Clamp;
    AddressV  = Clamp;

float viewportWidth;
float viewportHeight;

// Calculate the 2D screen position of a 3D position
float2 postProjToScreen(float4 position) {
    float2 screenPos = position.xy / position.w;
    return 0.5f * (float2(screenPos.x, -screenPos.y) + 1);

// Calculate the size of one half of a pixel, to convert
// between texels and pixels
float2 halfPixel() {
    return 0.5f / float2(viewportWidth, viewportHeight);

struct VertexShaderInput {
    float4 Position : POSITION0;

struct VertexShaderOutput {
    float4 Position :POSITION0;
    float4 PositionCopy : TEXCOORD1;

VertexShaderOutput VertexShaderFunction(VertexShaderInput input) {
    VertexShaderOutput output;
    output.Position = input.Position;
    return output;

float4 PixelShaderFunction(VertexShaderOutput input) : COLOR0 {
    float2 texCoord =postProjToScreen(input.PositionCopy) + halfPixel();

    // Extract the depth for this pixel from the depth map
    float4 depth = tex2D(depthSampler, texCoord);
    //return float4(depth.r,0,0,1);
    // Recreate the position with the UV coordinates and depth value
    float4 position;
    position.x = texCoord.x * 2 - 1;
    position.y = (1 - texCoord.y) * 2 - 1;
    position.z = depth.r;
    position.w = 1.0f;

    // Transform position from screen space to world space
    position = mul(position, InvViewProjection);
    position.xyz /= position.w;
    //compute projection
    float3 projection=tex2D(projectorSampler,postProjToScreen(mul(position,ProjectorViewProjection)) + halfPixel());
    return float4(projection,1);

In first part of pixel shader is recovered position from G-buffer (this code I am using in other shaders without any problem) and then is tranformed to projector viewprojection space. Problem is that projection doesn't appear. Here is an image of my situation:

image of problem

The green lines are the rendered projector frustum. Where is my mistake hidden? I am using XNA 4. Thanks for advice and sorry for my English.


Shader above is working but projection was too small. When I changed the Scale property to a large value (e.g. 100), the projection appears. But when the camera moves toward the projection, the projection expands, as can bee seen on this YouTube video.


In your shader, you'll need to remove the position.xyz /= w; as this is what is causing your resizing issue:

// Transform position from screen space to world space 
position = mul(position, InvViewProjection); 
position.xyz /= position.w;  <<-- Comment this out

That should do the trick.

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If you get results that are too small and get to that size it might be because the frustum is still in 1 to -1 space or 0 to 1 or the like? Is that your origo that the frustum is based around?

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