I'm getting some strange behavior where two seemingly equivalent expression in the fragment shader end up with totally different results.
I was able to condense the problem down to the test case below where "gl_FragColor = vec4(vec3(a * a *0.3), 1);" produces the result in the picture while "gl_FragColor = vec4(vec3(0.3 * a * a), 1);" works correctly and just outputs gray in the left half and black in the right half.
Looks like those artifacts happen around non uniform shader execution.
#include "GL/glew.h"
#include "GL/glut.h"
#include <vector>
const char* vs_source =
R"(
#version 130
void main(void)
{
gl_Position = gl_Vertex;
}
)";
const char* fs_source =
R"(
#version 130
void main(){
float y;
for(y = 1; y < 42; y++)
{
float x = gl_FragCoord.x;
if(x > 300) break;
}
float a = y / 43;
gl_FragColor = vec4(vec3(a * a *0.3), 1); // broken
//gl_FragColor = vec4(vec3(0.3 * a * a), 1); // works
}
)";
void printObjLog(GLuint obj){
GLint logLength = 0;
glGetObjectParameterivARB(obj, GL_OBJECT_INFO_LOG_LENGTH_ARB, &logLength);
if(logLength > 0){
std::vector<GLcharARB> errorLog(logLength,0);
glGetInfoLogARB(obj, logLength,0,&errorLog[0]);
fprintf(stderr, &errorLog[0]);
}
}
GLuint compileShader(GLuint type, const char* src)
{
GLint compile_ok = GL_FALSE;
GLuint shader = glCreateShader(type);
glShaderSource(shader, 1, &src, NULL);
glCompileShader(shader);
glGetShaderiv(shader, GL_COMPILE_STATUS, &compile_ok);
if (0 == compile_ok){
fprintf(stderr, "Error in shader:\n");
printObjLog(shader);
return 0;
}
return shader;
}
int init_resources(){
GLuint program = glCreateProgram();
GLuint vs = compileShader(GL_VERTEX_SHADER, vs_source);
GLuint fs = compileShader(GL_FRAGMENT_SHADER, fs_source);
glAttachShader(program, vs);
glAttachShader(program, fs);
GLint link_ok = GL_FALSE;
glLinkProgram(program);
glGetProgramiv(program, GL_LINK_STATUS, &link_ok);
if (!link_ok) {
fprintf(stderr, "glLinkProgram:");
printObjLog(program);
return 0;
}
glUseProgram(program);
return 1;
}
void onDisplay()
{
glClearColor(0.0, 0.0, 0.0, 0.0);
glClear(GL_COLOR_BUFFER_BIT);
glBegin(GL_TRIANGLES);
glVertex3f(-1,-1,0);
glVertex3f(1,-1,0);
glVertex3f(-1,1,0);
glEnd();
glBegin(GL_TRIANGLES);
glVertex3f(-1, -1, 0);
glVertex3f(4, -1, 0);
glVertex3f(-1, 4, 0);
glEnd();
glutReportErrors();
glutSwapBuffers();
}
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_RGBA|GLUT_DOUBLE|GLUT_DEPTH);
glutInitWindowSize(600, 600);
glutCreateWindow("gltestcase");
GLenum glew_status = glewInit();
if (glew_status != GLEW_OK)
{
fprintf(stderr, "Error: %s\n", glewGetErrorString(glew_status));
return EXIT_FAILURE;
}
if (1 == init_resources()) {
glutDisplayFunc(onDisplay);
glutMainLoop();
}
return 0;
}
