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OpenGL Screen space reflections: Issues

5 posts in this topic

Hey there gamedev people,

this is my firt post on the forums, even if I'm reading and searching here for some years now.

I'm studying computer visualistics in Germany and do a lot of OpenGL programming at the moment.

I'm currently trying to implement screen space reflections, but got some issues.

I first gather all informations in a G-Buffer (first pass rendering to frame buffer object) and do the lighting in a second pass.

Then I have a third pass for calculating the reflections.

Unfortunatelly, my reflections seem to be dependent of a certain viewing direction. They start to appear or disappear depending on the incident angle.

After implementing the approach several times and trying to understand the geometry on paper, this seems to be one of my last issues, until the reflections looks correct. I would be very happy if you'd take a lot at my code and hopefully someone can tell me my mistakes, I'd really appreciate that

Here's a short video to better show the problem:

http://youtu.be/4fyVNkr4wkY

/******************************************************************************/
// LATEST (working, with some perspective errors)
/* SSR (screen space reflections)
******************************************************************************/
vec4 SSR()
{
// Variables
vec4 fragmentColor = vec4(0.0f, 0.0f, 0.0f, 0.0f);
float initalStep = 0.001f;
float stepSize = 0.01f;
float blurSize = 1.0f;

// Current fragment
vec2 fragment = gl_FragCoord.xy/vec2(Screen.Width, Screen.Height);
vec2 ssfragment = 0.5f * fragment + 0.5f;
vec3 ssPosition = vec3(ssfragment, 0.0f);
ssPosition.z = linearizeDepth( texture(deferredDepthTex, vert_UV) );

// Normal & position
vec3 vsNormal = normalize(texture(deferredNormalTex, fragment).xyz);
vec3 vsPosition = texture(deferredPositionTex, fragment).xyz;

// View vector
vec3 vsViewVec = normalize( -ssPosition );
vsViewVec.y -= Camera.Position.y;

// Reflection vector
vec3 vsReflectVec = reflect(vsViewVec, vsNormal);
vsReflectVec = normalize(vsReflectVec);

// Initialze traced ray
vec3 initialRay = vsReflectVec * initalStep;

vec3 tracedRay = initialRay;
// Get depth informations
float fragmentDepth = linearizeDepth(texture(deferredDepthTex, fragment));
vec3 samplingPosition = ssPosition + tracedRay;
float sampledDepth = linearizeDepth(texture(deferredDepthTex, samplingPosition.xy));
float rayDepth =  ssPosition.z + tracedRay.z * fragmentDepth;

// Ray tracing while in screen space
int count = 0;
while(
samplingPosition.x > 0.0f && samplingPosition.x < 1.0f &&
samplingPosition.y > 0.0f && samplingPosition.y < 1.0f)
{
// Update sampling position and depth values
samplingPosition.x = (2.0f * ssPosition.x - 1.0f) + tracedRay.x;
samplingPosition.y = (2.0f * ssPosition.y - 1.0f) + tracedRay.y;
sampledDepth = linearizeDepth( texture(deferredDepthTex, samplingPosition.xy) );
rayDepth = ssPosition.z + tracedRay.z * fragmentDepth;

// intersection found
if(rayDepth > sampledDepth)
{
if(abs(rayDepth - sampledDepth) < 0.005f)
{
fragmentColor = vec4(
texture(deferredDiffuseTex, samplingPosition.xy).rgb, 1.0f );
break;
}
// Ray tracing termination
break;
}
else
fragmentColor = vec4(texture(deferredDiffuseTex, fragment).rgb, 1.0f);

tracedRay += tracedRay * stepSize;
count++;
}

// Return color from sampled fragment
return fragmentColor;
}


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You're definitely doing something wrong.

Basically your reflection rays are terribly wrong because reflections shouldn't transform when you rotate viewport (but they do). I'd say your screen space ray origins are wrong, shouldn't initial ray be viewVector * depth (although I'm not entirely sure, I'd probably have to test & go through your code at some better time than 4am)?

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I recently changed my code. I now calculate the reflected ray in the fragment shader of the g-Buffer using:

vec4 ReflectVec = normalize( reflect(vert_EyePosition, Normal) );

Where vert_EyePosition is the negative vertex position, which is multiplied with the model-view-matrix before in the vertex shader and

Normal is the interpolated normal from the vertex shader.

My resulting image of the reflected vector looks like this:

But acctually my tracing doesn't work now, so obviously my reflection vector wasn't correct before. Now I just have to get the tracing right...

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Okay, I got it working  , had' some space conversion problems. And adding the camera's near-plane value to the view vector helped very much:

Could you post the final working code as reference?
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