opengl: rendering to texture goes wrong - opengl

Question

I've written a program for testing rendering a texture with the framebuffer. In the function render_texture() I want to render a triangle on to a texture, but when I display the rendered texture in the display function, I get just a simple yellow square.
#include "GL/glew.h"
#include "GL/glext.h"
#include "GL/glu.h"
#include "GL/freeglut.h"
#include <cstdio>
uint16_t tex_width = 75;
uint16_t tex_height = 24;
GLuint texture;
int w1;
int h1;
void orthogonalStart (int w, int h) {
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
gluOrtho2D(0, w, 0, h);
glScalef(1, -1, 1);
glTranslatef(0, -h1, 0);
glMatrixMode(GL_MODELVIEW);
}
void orthogonalEnd (void) {
glMatrixMode(GL_PROJECTION);
glPopMatrix();
glMatrixMode(GL_MODELVIEW);
}
void display (void) {
glClearColor (0.0,0.0,0.0,1.0);
glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
orthogonalStart(w1, h1);
glBegin(GL_QUADS);
glTexCoord2f(0, 0); glVertex2f(125, 125);
glTexCoord2f(0, 1); glVertex2f(125, 125+tex_height);
glTexCoord2f(1, 1); glVertex2f(125+tex_width, 125+tex_height);
glTexCoord2f(1, 0); glVertex2f(125+tex_width, 125);
glEnd();
orthogonalEnd();
glutSwapBuffers();
}
void reshape (int w, int h) {
glViewport (0, 0, (GLsizei)w, (GLsizei)h);
glMatrixMode (GL_PROJECTION);
glLoadIdentity ();
gluPerspective (60, (GLfloat)w / (GLfloat)h, 0.1, 1000.0);
w1 = w;
h1 = h;
glMatrixMode (GL_MODELVIEW);
}
void render_texture()
{
GLuint framebuffer = 0;
glGenFramebuffers(1, &framebuffer);
glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);
glGenTextures(1, &texture);
glBindTexture(GL_TEXTURE_2D, texture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, tex_width, tex_height, 0, GL_RGBA,
GL_UNSIGNED_BYTE, 0);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texture, 0);
GLenum draw_buffers[1] = {GL_COLOR_ATTACHMENT0};
glDrawBuffers(1, draw_buffers);
if(glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE)
{
fprintf(stderr, "[render_texture] Fehler im Framebuffer\n");
exit(1);
}
glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);
glViewport(0, 0, tex_width, tex_height);
glClearColor (0.0,0.0,0.0,1.0);
orthogonalStart(tex_width, tex_height);
glColor4f(1, 1, 0, 0);
glBegin(GL_TRIANGLES);
glVertex2f(0.f,0.f);
glVertex2f(tex_width, 0.0f);
glVertex2f(tex_width, (GLfloat)tex_height/2.f);
//glVertex2f(tex_width-(GLfloat)tex_height/2.f, tex_height);
//glVertex2f(0, tex_height);
glEnd();
orthogonalEnd();
glBindFramebuffer(GL_FRAMEBUFFER, 0);
}
int main (int argc, char **argv) {
glutInit (&argc, argv);
glutInitDisplayMode (GLUT_DOUBLE | GLUT_DEPTH | GLUT_RGBA);
glutInitWindowSize (500, 500);
glutInitWindowPosition (100, 100);
glutCreateWindow ("");
glutDisplayFunc (display);
glutIdleFunc (display);
glutReshapeFunc (reshape);
glewExperimental=true;
GLenum err=glewInit();
if(err!=GLEW_OK)
{
//Problem: glewInit failed, something is seriously wrong.
fprintf(stderr, "glewInit failed, aborting.\n");
}
render_texture();
glutMainLoop ();
return 0;
}
Edit 1: I've updated the code, now I have just a blank black screen
#include "GL/glew.h"
#include "GL/glext.h"
#include "GL/glu.h"
#include "GL/freeglut.h"
#include <cstdio>
uint16_t tex_width = 75;
uint16_t tex_height = 24;
GLuint texture;
void orthogonalStart (int w, int h) {
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
gluOrtho2D(0, w, 0, h);
glScalef(1, -1, 1);
glTranslatef(0, -glutGet(GLUT_WINDOW_HEIGHT), 0);
glMatrixMode(GL_MODELVIEW);
}
void orthogonalEnd (void) {
glMatrixMode(GL_PROJECTION);
glPopMatrix();
glMatrixMode(GL_MODELVIEW);
}
void display (void) {
glClearColor (0.0,0.0,0.0,1.0);
glClear (GL_COLOR_BUFFER_BIT);
glLoadIdentity();
orthogonalStart(glutGet(GLUT_WINDOW_WIDTH), glutGet(GLUT_WINDOW_HEIGHT));
glColor3f(1,1,1);
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, texture);
glBegin(GL_QUADS);
glTexCoord2f(0, 0); glVertex2f(125, 125);
glTexCoord2f(0, 1); glVertex2f(125, 125+tex_height);
glTexCoord2f(1, 1); glVertex2f(125+tex_width, 125+tex_height);
glTexCoord2f(1, 0); glVertex2f(125+tex_width, 125);
glEnd();
glDisable(GL_TEXTURE_2D);
orthogonalEnd();
glutSwapBuffers();
}
void reshape (int w, int h) {
glViewport (0, 0, (GLsizei)w, (GLsizei)h);
glMatrixMode (GL_PROJECTION);
glLoadIdentity ();
gluPerspective (60, (GLfloat)w / (GLfloat)h, 0.1, 1000.0);
glMatrixMode (GL_MODELVIEW);
}
void render_texture()
{
GLuint framebuffer = 0;
glGenFramebuffers(1, &framebuffer);
glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);
glGenTextures(1, &texture);
glBindTexture(GL_TEXTURE_2D, texture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, tex_width, tex_height, 0, GL_RGBA,
GL_UNSIGNED_BYTE, 0);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texture, 0);
GLenum draw_buffers[1] = {GL_COLOR_ATTACHMENT0};
glDrawBuffers(1, draw_buffers);
if(glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE)
{
fprintf(stderr, "[render_texture] Fehler im Framebuffer\n");
exit(1);
}
glViewport(0, 0, tex_width, tex_height);
glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);
orthogonalStart(tex_width, tex_height);
glColor4f(1, 1, 0, 0);
glBegin(GL_TRIANGLES);
glVertex2f(0.f,0.f);
glVertex2f(tex_width, 0.0f);
glVertex2f(tex_width, (GLfloat)tex_height/2.f);
//glVertex2f(tex_width-(GLfloat)tex_height/2.f, tex_height);
//glVertex2f(0, tex_height);
glEnd();
orthogonalEnd();
glBindFramebuffer(GL_FRAMEBUFFER, 0);
}
int main (int argc, char **argv) {
glutInit (&argc, argv);
glutInitContextVersion(3, 1);
glutInitContextProfile(GLUT_CORE_PROFILE);
glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGBA);
glutInitWindowSize (500, 500);
glutCreateWindow ("");
glutDisplayFunc (display);
glutIdleFunc (display);
glutReshapeFunc (reshape);
glewExperimental=true;
GLenum err=glewInit();
if(err!=GLEW_OK)
{
//Problem: glewInit failed, something is seriously wrong.
fprintf(stderr, "glewInit failed, aborting.\n");
}
//glEnable(GL_TEXTURE_2D);
render_texture();
glutMainLoop ();
return 0;
}

Answer 1

You are clearly doing something wrong in the setup of your projection matrix.
I'm not sure why you are calling glScalef() and glTranslatef(), if you comment out these lines in your orthogonalStart() function, you'll have your triangle.
void orthogonalStart (int w, int h) {
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
gluOrtho2D(0, w, 0, h);
//glScalef(1, -1, 1);
//glTranslatef(0, -glutGet(GLUT_WINDOW_HEIGHT), 0);
glMatrixMode(GL_MODELVIEW);
}
On a sidenote, there is no need to call glBindFramebuffer(GL_FRAMEBUFFER, framebuffer) a 2nd time in your render_texture() function call.
Also, I would recommend you to start learning about vertex buffers to draw your objects, which is more elaborated here.