View Full Version : How to pass float data into the fragment shader and back out in openGL ES?

12-13-2016, 10:46 PM
Hi, I have been studying openGL ES for a week now, and I am not sure how to get float data in and out using FBO. Here are my C codes which gives no error:

/* allocate CPU memory */
float *dens = (float *) malloc (4*(W)*(H)*sizeof(float));
float* result = (float*) malloc (4*W*H*sizeof(float));

for (int i = 0; i < 4*W*H; i++) {
dens[i] = 10;
result[i] = 0;
/* create FBO */
GLuint FBO;
glGenFramebuffers(1, &FBO);
glBindFramebuffer(GL_FRAMEBUFFER, FBO);

/* create input texture */
GLuint dens_gl;
glGenTextures(1, &dens_gl);
glBindTexture(GL_TEXTURE_2D, dens_gl);


glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA16F, W, H, 0, GL_RGBA, GL_FLOAT, dens);

GLint Loc;
Loc = glGetUniformLocation(_program, "dens_gl");
glUniform1i(Loc, 0);

glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, dens_gl, 0);

/* read GPU memory to result pointer */
glReadPixels(0, 0, W, H,GL_RGBA,GL_FLOAT, result);

I have already set the vertex shader to the corners so my fragment shader becomes pixel shader. I then try to do some calculations in fsh:

uniform sampler2D dens_gl;
void main(void) {
float d = texture2D(dens_gl, uv).r;
d = d*0.5;
gl_FragColor = vec4(d,d,d,1.0);

but the result pointer at the end only holds the same value as the dens CPU memory ( from 0 changed to 10). What did I do wrong? I have used glgeterror function everywhere and I do not receive an error.

Dark Photon
12-14-2016, 05:29 AM
...but the result pointer at the end only holds the same value as the dens CPU memory ( from 0 changed to 10). What did I do wrong?

You might post a short, standalone test program that folks can compile and run locally on their PC and use to spot things that aren't in your snippet (such as a bind of the program object and a draw call, both of which aren't present in your snippet). You might also check that glGetUniformLocation is returning a non-negative value.

I also don't see an explicit set of GL_PACK_ALIGNMENT, but in your case the default value of 4 should be just as good as 1.