Hi everyone,

I’m making changes to my deferred system. I want to delete the position texture (too much expensive GL_RGB32F) so in the light pass I have to reconstruct the pixel position from depth.

I tried for a few days without luke, so now I’m some how frustated.

What works (old position texture):

Geometry pass - Vertex:

```
vsPosition = ( ModelViewMatrix * vec4(in_Position, 1.0) ).xyz;
...
gl_Position = MVP * vec4(in_Position, 1.0);
```

Geometry pass - Fragment:

```
fsPosition = vsPosition;
```

This is how I store old pixel positions on geometry pass.

So next is to figure how to get this without the texture. I tried MANY things that I found around the net (without luke…), what i have now is:

Light pass - fragment:

```
vec2 calcTexCoord()
{
return gl_FragCoord.xy / ScreenSize;
}
vec3 positionFromDepth()
{
vec2 sp = calcTexCoord();
float depth = texture2D(Texture4, sp).x * 2.0 - 1.0;
vec4 pos = InvProjectionMatrix * vec4( sp*2.0-1.0, depth, 1.0);
return pos.xyz/pos.w;
}
vec3 computePhongPointLight()
{
vec3 color = vec3(0.0);
vec2 texCoord = calcTexCoord();
//vec3 position = texture2D( Texture0, texCoord ).xyz;
vec3 position = positionFromDepth();
vec3 difColor = texture2D( Texture1, texCoord ).xyz;
vec3 specColor = texture2D( Texture2, texCoord ).xyz;
vec3 normColor = texture2D( Texture3, texCoord ).xyz;
vec3 lightDir = Light.position.xyz - position;
vec3 lightDirNorm = normalize( lightDir );
float sDotN = max( dot(lightDirNorm, normColor), 0.0);
float att = 1.0;
float distSqr = dot(lightDir, lightDir);
float invAtt = (Light.constantAtt +
(Light.linearAtt*sqrt(distSqr)) +
(Light.quadraticAtt*distSqr));
att = 0.0;
if (invAtt != 0.0)
att = 1.0/invAtt;
vec3 diffuse = difColor.rgb * Light.diffuse * sDotN;
vec3 ambient = difColor.rgb * Light.ambient; // Cheat here
vec3 vertexToEye = normalize(position);
vec3 r = normalize(reflect(lightDirNorm, normColor));
// SpecularPower
vec3 specular = vec3(0.0);
if ( sDotN > 0.0 )
{
specular = Light.specular.rgb *
specColor *
pow( max( dot(vertexToEye, r), 0.0), 60.0 ); // Change specular here!!!! value 60 must be an uniform
}
return (diffuse + specular + ambient)*att;
}
```

Where Texture0 is old position texture, Texture4 is depth texture, InvProjectionMatrix is the inverse projection matrix, and Light.position is computed as ViewMatrix * light_position.

I did some debug and I output the absolute diference:

```
vec3 pos1 = positionFromDepth();
vec3 pos2 = texture2D(Texture0, calcTexCoord()).xyz;
fragColor = vec4(abs(pos2.x-pos1.x), abs(pos2.y-pos1.y), abs(pos2.z-pos1.z), 1.);
```

The output is all black minus empty zones that are white.

I think it’s space error, something like: light position in view space and pixel position is in other space so “vec3 lightDir = Light.position - position” give bad results. But I can’t figure what’s happening, and how to solve it…

I will really apreciate any help (cause I don’t know what more to do), and I hope that someone with better math can help me

Sorry for my bad English and thanks in advance.