Offscreen rendered texture renders empty rectangle on screen

Question

I'm trying to create OpenGL ES 1.1 texture and fill is with rendered content.

Preparing a texture:

glGenTextures(1, &texture);
glBindTexture(GL_TEXTURE_2D, texture);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 32, 32, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);

GLuint texFrambuffer;
glGenFramebuffers(1, &texFrambuffer);
glBindFramebuffer(GL_FRAMEBUFFER, texFrambuffer);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texture, 0);

GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER) ;
if(status != GL_FRAMEBUFFER_COMPLETE) {
    NSLog(@"failed to make complete framebuffer object %x", status);
    // this message doesn't appear, thus, framebuffer is complete
}

glViewport(0, 0, 32, 32);

glClearColor(1.0f, 0.0f, 1.0f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);

GLfloat vertices[] = {
    10.0f, 10.0f, 0.0f,
    20.0f, 40.0f, 0.0f,
    10.0f, 30.0f, 0.0f
};
glColor4f(0.0f, 1.0f, 0.0f, 1.0f);
glVertexPointer(3, GL_FLOAT, 0, vertices);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 3);

glBindFramebuffer(GL_FRAMEBUFFER, 0);
glDeleteFramebuffers(1, &texFrambuffer);

Rendering texture into scene:

glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, texture);

glColor4f(1.0f, 1.0f, 1.0f, 1.0f);
GLshort texCoord[] = {0, 0, 0, 1, 1, 0, 1, 1};
GLfloat vertices2[] = {0.0, 0.0, 0.0, 31.0f, 31.0f, 0.0f, 31.0f, 31.0f};
glVertexPointer(2, GL_FLOAT, 0, vertices2);
glTexCoordPointer(2, GL_SHORT, 0, texCoord);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

glDisable(GL_TEXTURE_2D);

Full source code is available at Bitbucket: https://bitbucket.org/amjaliks/gl2/src/66ee4932e212/GL2/GL2View.m

There other content is rendered correctly. Issue is only with this texture – it renders as a white rectangle.

I have studied a lot of example, but still can't figure out what I doing wrong. What is missing?

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Update 1: I have noticed, if I set glClearColor() and call glClear() while texFrambuffer is bound, these changes is saved to my texture. Thus, there is something wrong with glDrawArrays() and/or related things.

Do I need set matrix mode and projection specifically for my texFrambuffer?

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Update 2: I have tried also to use glCopyTexImage2D to copy content from "main" framebuffer to a texture.

glGenTextures(1, &texture);
glBindTexture(GL_TEXTURE_2D, texture);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 32, 32, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);

glViewport(0, 0, 32, 32);

glClearColor(1.0f, 0.75f, 0.5f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);
GLfloat vertices[] = {
    0.0f, 0.0f, 0.0f,
    20.0f, 32.0f, 0.0f,
    10.0f, 30.0f, 0.0f
};
glColor4f(0.0f, 1.0f, 0.0f, 1.0f);
glVertexPointer(3, GL_FLOAT, 0, vertices);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 3);

glBindTexture(GL_TEXTURE_2D, texture);
glCopyTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 0, 0, 32, 32, 0);

Full source code available at Bitbucket: https://bitbucket.org/amjaliks/gl2/src/b3339661650e/GL2/GL2View.m

The result is the same. Only filled rectangle. It looks like there is definitely issue with my matrices.

Answer 1

Your problem is with matrices. By default projection matrix is identity. Thus giving (-1,-1)-(+1,+1) coordinates for viewport. Most likely you need orthographic projection to (0,0)-(32,32) square.