opengl render to texture just see a black region

Question

I am having some trouble in OpenGL, making a render to texture example work. At initialization, i generate a texture 'randtex' with random values of green and black. If i render this texture directly to the window (mapped into a quad) it works all well. like this:

But if i render 'randtex' into another texture 'tex' which is attached to a framebuffer object, then rendering 'tex' on the screen just gives me a black image on the fbo's blue background and from what i know it should give me the original texture over the blue background. In other words, this is what i am getting

vertex shader for display only (display_shaderp).

#version 420
in vec4 pos;
in vec2 tex_coord;
out vec2 vtex_coord;
uniform mat4 projection;
uniform mat4 modelview;
void main(){
gl_Position = projection * modelview * pos;
    vtex_coord = tex_coord;
}

fragment shader for display only (display_shaderp)

#version 420
in vec2 vtex_coord;
uniform sampler2D tex;
out vec4 color;
void main(){
    color = texture2D(tex, vtex_coord);
    //color = vec4(1.0f, 1.0f, 1.0f, 1.0f);
}

shader program compiles and links ok, i get no gl errors and framebuffer is complete without errors too. This is the rendering code:

glClearColor(0.5, 0.5, 0.5, 1.0);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glViewport(0, 0, win_width, win_height);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(60.0f, (GLfloat)win_width / (GLfloat) win_height, 0.1f, 50.0f);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslate3f(0.0f, 0.0f, -3.0f)

// render to texture
glBindFramebuffer(GL_FRAMEBUFFER, fbo);
glViewport(0,0, win_width, win_height);
glClearColor(0.0, 0.0, 0.5, 1.0);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glUseProgram(fbo_shaderp);
GLfloat m_matrix[16], p_matrix[16];
glGetFloatv(GL_MODELVIEW_MATRIX, m_matrix); 
glGetFloatv(GL_PROJECTION_MATRIX, p_matrix);
glUniformMatrix4fv(glGetUniformLocation(fbo_shaderp, "modelview"),1,GL_FALSE,m_matrix);
glUniformMatrix4fv(glGetUniformLocation(fbo_shaderp, "projection"),1,GL_FALSE,p_matrix);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, randtex);
glUniform1i(glGetUniformLocation(fbo_shaderp, "tex"), randtex);
GLuint _p = glGetAttribLocation(fbo_shaderp, "pos");
GLuint _t = glGetAttribLocation(fbo_shaderp, "tex_coord");
glVertexAttribPointer(_p, 3, GL_FLOAT,  GL_FALSE, 5 * sizeof(GLfloat), 0);
glVertexAttribPointer(_t, 2, GL_FLOAT,  GL_FALSE, 5 * sizeof(GLfloat), (GLvoid*)(sizeof(float)*3));
glEnableVertexAttribArray(_p);
glEnableVertexAttribArray(_t);
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_BYTE, 0);
glUseProgram(0);    
glBindFramebuffer(GL_FRAMEBUFFER, 0);

    // render to the window
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glViewport(0,0, win_width, win_height);
glClearColor(0.5, 0.5, 0.5, 1.0);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glUseProgram(display_shaderp);
glUniformMatrix4fv(glGetUniformLocation(display_shaderp,"modelview"),1,GL_FALSE,m_matrix);
glUniformMatrix4fv(glGetUniformLocation(display_shaderp,"projection"),1,GL_FALSE,p_matrix);
glActiveTexture(GL_TEXTURE0); 
glBindTexture(GL_TEXTURE_2D, tex); 
glUniform1i(glGetUniformLocation(display_shaderp, "tex"), 0);
GLuint _p = glGetAttribLocation(display_shaderp, "pos");
GLuint _t = glGetAttribLocation(display_shaderp, "tex_coord");
glVertexAttribPointer ( _p, 3, GL_FLOAT, GL_FALSE, 5 * sizeof(GLfloat), 0 );
glVertexAttribPointer ( _t, 2, GL_FLOAT, GL_FALSE, 5 * sizeof(GLfloat), (GLvoid*) (sizeof(float) * 3) );
glEnableVertexAttribArray(_p);
glEnableVertexAttribArray(_t);
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_BYTE, 0);
glUseProgram(0);

And the code to create textures and framebuffer

int i = 0;
// create a random texture 'randtex'
glGenTextures(1, &randtex);
glBindTexture(GL_TEXTURE_2D, randtex);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
// random data
GLubyte* data = (GLubyte *) malloc(width*height*4*sizeof(GLubyte));
GLubyte val;
for (i = 0; i < width * height * 4; i+=4){
    if ((double)rand()/(double)RAND_MAX > 0.8)
        val = 255;
    else
        val = 0;
    data[i]   = 0;
    data[i+1] = val;
    data[i+2] = 0;
    data[i+3] = 255;
}
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA,GL_UNSIGNED_BYTE, data);
// create an empty texture 'tex'
glGenTextures(1, &tex);
glBindTexture(GL_TEXTURE_2D, tex);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA,GL_UNSIGNED_BYTE, 0);
// create framebuffer and attach 'tex'
glGenFramebuffers(1, &fbo);
glBindFramebuffer(GL_FRAMEBUFFER, fbo);
glFramebufferTexture2D(GL_FRAMEBUFFER,GL_COLOR_ATTACHMENT0,GL_TEXTURE_2D, tex, 0);
GLenum status;
if ((status = glCheckFramebufferStatus(GL_FRAMEBUFFER)) != GL_FRAMEBUFFER_COMPLETE)
    fprintf(stderr, "glCheckFramebufferStatus: error %p", status);

shaders for render to texture (fbo_shaderp)

render to texture vertex shader

in vec4 pos;
in vec2 tex_coord;
out vec2 vtex_coord;
uniform mat4 projection;
uniform mat4 modelview;
void main(){
    gl_Position = projection * modelview * pos;
    vtex_coord = tex_coord;
}

render to texture fragment shader

#version 420
in vec2 vtex_coord;
layout(location = 0) out vec4 color;
uniform sampler2D tex;

void main(){
color = texture2D(tex, vtex_coord);
//color = vec4(1.0f, 1.0f, 1.0f, 1.0f);
}

In this last shader, if i use the commented line to paint all white and comment out the texture one, i do get a white image but also opengl error right after rendering to texture "OpenGL Error: invalid value", so this actually confuses me more.

Answer 1

glUniform1i(glGetUniformLocation(fbo_shaderp, "tex"), randtex);

You must not give the ID of the texture, but the slot you bind the texture to. So in your case, that should be

glUniform1i(glGetUniformLocation(fbo_shaderp, "tex"), 0);