I have checked this similar question, but the suggestions did not solve my problem: Low precision and clamping when writing to floating point FBO.
I am rendering a float image using a framebuffer object as render target. However, the values read by glReadPixels are clamped between 0 and 1. If I try to render my image using normalized values in this range, it works, but values outside this range are clamped to 1 or 0. Follows some parts of the code.
Here is how I created the texture that I attached to the FBO:
glGenTextures(1, &color_tex);
glBindTexture(GL_TEXTURE_RECTANGLE_ARB, color_tex);
glTexParameteri(GL_TEXTURE_RECTANGLE_ARB, GL_TEXTURE_MIN_FILTER,
GL_NEAREST);
glTexParameteri(GL_TEXTURE_RECTANGLE_ARB, GL_TEXTURE_MAG_FILTER,
GL_NEAREST);
glTexImage2D(GL_TEXTURE_RECTANGLE_ARB, 0, GL_RGB32F_ARB, ResX, ResY, 0,
GL_RGB, GL_FLOAT, NULL);
How I read it after painting:
glReadBuffer(GL_COLOR_ATTACHMENT0_EXT);
glReadPixels(0, 0, ResX, ResY, GL_RGB, GL_FLOAT, data);
I have painted a triangle in the image using
glColor3f(0.5f, 2.0f, -0.5f);
And the pixel values appears as (0.5f, 1.0f, 0.0f) in the read buffer.
How can I return values out of this range?
Follows a simplified version of my code (I kept the depth buffer because I use it in the original one):
Ps. I corrected the code bellow based on the answer. It works now.
#include <GL/glew.h>
#include <GL/glut.h>
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#define WINDOW_WIDTH 800
#define WINDOW_HEIGHT 600
bool initGL(int argc, char *argv[])
{
// initialize GLUT and glexts
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
glutInitWindowSize(WINDOW_WIDTH, WINDOW_HEIGHT);
glutInitWindowPosition(100, 100);
glutCreateWindow("FBO");
GLenum err = glewInit();
if (GLEW_OK != err)
{
/* Problem: glewInit failed. */
printf("Error: %s\n", glewGetErrorString(err));
return 0;
}
if (!GLEW_EXT_framebuffer_object)
{
printf("Error: no extension GL_EXT_framebuffer_object.");
return 0;
}
if (!GLEW_ARB_color_buffer_float)
{
printf("Error: no extension ARB_color_buffer_float.");
return 0;
}
glClampColor(GL_CLAMP_READ_COLOR, GL_FALSE);
glClampColor(GL_CLAMP_VERTEX_COLOR, GL_FALSE);
glClampColor(GL_CLAMP_FRAGMENT_COLOR, GL_FALSE);
return 1;
}
void glErrorCheck()
{
int errCode;
if ((errCode = glGetError()) != GL_NO_ERROR)
{
printf("Failure in OpenGL %d", errCode);
exit(0);
}
}
bool renderFBO()
{
GLenum status;
GLuint color_tex, depth_rb, fb;
int ResX, ResY;
ResY = 4;
ResX = 4;
float *data;
//gen renderbuffer
glGenRenderbuffers(1, &depth_rb);
glErrorCheck();
// initialize depth renderbuffer
glBindRenderbuffer(GL_RENDERBUFFER, depth_rb);
glErrorCheck();
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT32, ResX,
ResY);
glErrorCheck();
// make a texture
glGenTextures(1, &color_tex);
// initialize texture that will store the framebuffer image (BGRA type)
glBindTexture(GL_TEXTURE_RECTANGLE_ARB, color_tex);
glTexParameteri(GL_TEXTURE_RECTANGLE_ARB, GL_TEXTURE_MIN_FILTER,
GL_NEAREST);
glTexParameteri(GL_TEXTURE_RECTANGLE_ARB, GL_TEXTURE_MAG_FILTER,
GL_NEAREST);
glTexParameteri(GL_TEXTURE_RECTANGLE_ARB, GL_TEXTURE_WRAP_S, GL_CLAMP);
glTexParameteri(GL_TEXTURE_RECTANGLE_ARB, GL_TEXTURE_WRAP_T, GL_CLAMP);
glTexImage2D(GL_TEXTURE_RECTANGLE_ARB, 0, GL_RGB32F_ARB, ResX, ResY, 0,
GL_RGB, GL_FLOAT, NULL);
glErrorCheck();
// gen the framebuffer object
glGenFramebuffers(1, &fb);
// bind the framebuffer, fb, so operations will now occur on it
glBindFramebuffer(GL_FRAMEBUFFER, fb);
// bind this texture to the current framebuffer obj. as color_attachment_0
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
GL_TEXTURE_RECTANGLE_ARB, color_tex, 0);
glErrorCheck();
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT,
GL_RENDERBUFFER, depth_rb);
glErrorCheck();
//check framebuffer status
status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
if (status != GL_FRAMEBUFFER_COMPLETE)
{
glBindFramebuffer(GL_FRAMEBUFFER, 0);
{
printf("Failure in OpenGL Framebuffer %d", status);
return 0;
}
}
glErrorCheck();
//render to GL_TEXTURE_2D
glBindFramebuffer(GL_FRAMEBUFFER, fb);
glClearColor(0.0, 0.0, 0.0, 1.0);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glViewport(0, 0, ResX, ResY);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0, (float) ResX, 0.0, (float) ResY, -1.0, 1.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glBegin(GL_TRIANGLES);
printf("Original color: %f %f %f\n", 0.5f, 2.0f, -0.5f);
glColor3f(0.5f, 2.0f, -0.5f);
glVertex3f(0.0f, 0.0f, 0.0f);
glVertex3f((float) ResX, 0.0f, 0.0f);
glVertex3f((float) ResX, (float) ResY, 0.0f);
glEnd();
// read framebuffer
glReadBuffer(GL_COLOR_ATTACHMENT0);
glErrorCheck();
// allocate memory for texture data
data = new float[ResX * ResY * 3];
if (data == NULL)
{
printf("Out of memory\n");
return false;
}
//glClampColor(GL_CLAMP_FRAGMENT_COLOR, GL_FALSE);
glClampColor(GL_CLAMP_READ_COLOR, GL_FALSE);
glReadPixels(0, 0, ResX, ResY, GL_RGB, GL_FLOAT, data);
glErrorCheck();
//print the last pixel of the buffer
int i = ResX * ResY - 1;
printf("Buffer color: %f %f %f\n", data[i], data[i + 1], data[i + 2]);
// Re-enable rendering to the window
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glErrorCheck();
//deallocate data
delete[] data;
//delete buffers
glDeleteTextures(1, &color_tex);
glDeleteRenderbuffers(1, &depth_rb);
glDeleteFramebuffers(1, &fb);
glErrorCheck();
return true;
}
int main(int argc, char **argv)
{
if (!initGL(argc, argv))
{
printf("Failure during GLUT/GLEXT initialization.");
return 0;
}
if (!renderFBO())
printf("Fail!\n");
printf("End!\n");
return 0;
}
