OpenGL pixel shader: Difference between revisions

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Getting a true (pseudo) random number is surprisingly tricky. The following code makes something noisy, but not at all random:

<lang c>#include <stdio.h>

#include <stdlib.h>

#include <GL/glew.h>

#include <GL/glut.h>

⚫

GLuint ps, vs, prog, r_mod;

float angle = 0;

void render(void)

{

glClear(GL_COLOR_BUFFER_BIT);

glUniform1f(r_mod, rand() / (float)RAND_MAX);

glLoadIdentity();

glRotatef(angle, .3, 1, 0);

glRotatef(angle, angle * .1, 1, 0);

glBegin(GL_TRIANGLES);

glVertex3f(-.5, -.5, 0);

glVertex3f(-1, -.5, 0);

glVertex3f(0, .5, 0);

glVertex3f(0, 1, 0);

glVertex3f(.5, 0, 0);

glVertex3f(1, 0, 0);

glEnd();

angle += .01;

angle += .02;

glutSwapBuffers();

}

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void set_shader()

{

⚫

GLuint ps, vs, prog;

const char *f =

"varying float x, y, z;"

"uniform float r_mod;"

"float rand(float s) { return mod(abs(s) * 7654321 + 1, 1); }"

"~~void~~ ~~main~~() { ~~gl_FragColor~~ ~~= vec4~~(~~rand~~(~~x), rand~~(y), ~~rand(z~~), 1); }";

"float rand(float s) { return mod(pow(abs(s * 7.654321) + 1, 1 + r_mod), 1); }"

"void main() {"

" gl_FragColor = vec4(rand(gl_FragCoord.x + x), rand(gl_FragCoord.y + y), rand(gl_FragCoord.z + z), 1);"

"}";

const char *v =

"varying float x, y, z;"

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" gl_Position = ftransform();"

" x = gl_Position.x; y = gl_Position.y; z = gl_Position.z;"

" x *= z; y *= x; z += (x + y);"

" x += y; y -= x; z += x - y;"

"}";

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glLinkProgram(prog);

glUseProgram(prog);

r_mod = glGetUniformLocation(prog, "r_mod");

}

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glutInit(&argc, argv);

glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);

glutInitWindowPosition(0, 0);

glutInitWindowSize(200, 200);

glutCreateWindow("Stuff");

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if (!glewIsSupported("GL_VERSION_2_0")) {

fprintf(stderr, "GL 2.0 unsupported\n");

~~exit(~~1);

return 1;

}