Hello. I have encountered a problem when GLSL shader generates incorrect image on following GPU’s:
GT 430
GT 770
GTX 570
GTX 760
But works normally on these:
Intel HD Graphics 2500
Intel HD 4000
Intel 4400
GTX 740M
Radeon HD 6310M
Radeon HD 8850
Shader code is as follows:
bool PointProjectionInsideTriangle(vec3 p1, vec3 p2, vec3 p3, vec3 point)
{
vec3 n = cross((p2 - p1), (p3 - p1));
vec3 n1 = cross((p2 - p1), n);
vec3 n2 = cross((p3 - p2), n);
vec3 n3 = cross((p1 - p3), n);
float proj1 = dot((point - p2), n1);
float proj2 = dot((point - p3), n2);
float proj3 = dot((point - p1), n3);
if(proj1 > 0.0)
return false;
if(proj2 > 0.0)
return false;
if(proj3 > 0.0)
return false;
return true;
}
struct Intersection
{
vec3 point;
vec3 norm;
bool valid;
};
Intersection GetRayTriangleIntersection(vec3 rayPoint, vec3 rayDir, vec3 p1, vec3 p2, vec3 p3)
{
vec3 norm = cross(p1 - p2, p1 - p3);
norm /= (length(norm) + 1e-5);
Intersection res;
res.norm = norm;
res.point = vec3(rayPoint.xy, 0.0);
res.valid = PointProjectionInsideTriangle(p1, p2, p3, res.point);
return res;
}
#define raysCount 15
void main(void)
{
vec2 radius = (gl_FragCoord.xy / vec2(800.0, 600.0)) - vec2(0.5, 0.5);
struct ColoredIntersection
{
Intersection geomInt;
vec4 color;
};
ColoredIntersection ints[raysCount];
vec3 randomPoints[raysCount];
int i, j;
for(int i = 0; i < raysCount; i++)
{
float theta = 0.5 * float(i);
float phi = 3.1415 / 2.0;
float r = 1.0;
randomPoints[i] = vec3(r * sin(phi) * cos(theta), r * sin(phi)*sin(theta), r * cos(phi));
vec3 tangent = cross(vec3(0.0, 0.0, 1.0), randomPoints[i]);
tangent /= (length(tangent) + 1e-5);
vec3 trianglePoint1 = randomPoints[i] * 2.0 + tangent * 0.2;
vec3 trianglePoint2 = randomPoints[i] * 2.0 - tangent * 0.2;
ints[i].geomInt = GetRayTriangleIntersection(vec3(radius, -10.0), vec3(0.0, 0.0, 1.0), vec3(0.0, 0.0, 0.0), trianglePoint1, trianglePoint2);
if(ints[i].geomInt.valid) //1
{
float c = length(ints[i].geomInt.point);
ints[i].color = vec4(c, c, c, 1.0);
}
}
for(i = 0; i < raysCount; i++) //2
{
for(j = i + 1; j < raysCount; j++)
{
if(ints[i].geomInt.point.z < ints[i].geomInt.point.z - 10.0)
{
gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
return;
ColoredIntersection tmp = ints[j];
ints[j] = ints[i];
ints[i] = tmp;
}
}
}
vec4 resultColor = vec4(0.0, 0.0, 0.0, 0.0);
for(i = 0; i < raysCount + 0; i++)
{
if(ints[i].geomInt.valid)
resultColor += ints[i].color;
}
gl_FragColor = resultColor;
}
The code is a simplified version of an actual shader, expected image is:
External Media
But what I get is:
External Media
Random rotations of the code remove artifacts completely. For example if I change the line
if(ints[i].geomInt.valid) //1
to
if(ints[i].geomInt.valid == 0) //1
or completely remove double cycle that does nothing (marked as 2) artifacts vanish. Please note that the double cycle does nothing at all since condition
if(ints[i].geomInt.point.z < ints[i].geomInt.point.z - 10.0)
{
gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
return;
ColoredIntersection tmp = ints[j];
ints[j] = ints[i];
ints[i] = tmp;
}
Can never be satisfied(left and right sides have index i, not i, j) and there’s no NaN’s. This code does absolutely nothing yet somehow produces artifacts.
You can test the shader and demo on your own using this project(full MSVS 2010 project + sources + compiled binary and a shader, uses included SFML): https://dl.dropboxusercontent.com/u/25635148/ShaderTest.zip
I use sfml in this test project, but that’s 100% irrelevant because the actual project I’ve enountered this problem does not use this lib.
What I want to know is why these artifacts appear and how to reliably avoid them.