ios - OpenGL ES 2.0 render to texture

Question

I'm trying to render to a texture using OpenGL ES 2.0, but I can't seem to make it work.

This is how I proceed:

    struct RenderTexture
    {
        GLuint framebuffer;
        GLuint tex;
        GLint old_fbo;


        RenderTexture(GLuint width, GLuint height)
        {
            glGetIntegerv(GL_FRAMEBUFFER_BINDING, &old_fbo);

            glGenFramebuffers(1, &framebuffer);
            glGenTextures(1, &tex);

            glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);
            glBindTexture(GL_TEXTURE_2D, tex);
            glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 
                         width, height, 0, GL_RGBA, 
                         GL_UNSIGNED_BYTE, NULL);
            glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, 
                                   tex, 0);

            glClearColor(1, 0, 0, 1);
            glClear(GL_COLOR_BUFFER_BIT);

            GLuint status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
            if (status != GL_FRAMEBUFFER_COMPLETE) {
                cout << status << endl; // this is not called
            }

            glBindFramebuffer(GL_FRAMEBUFFER, old_fbo);
        }

        void begin()
        {
            glGetIntegerv(GL_FRAMEBUFFER_BINDING, &old_fbo);
            glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);
        }

        void end()
        {
            glBindFramebuffer(GL_FRAMEBUFFER, old_fbo);
        }
    };

But when I try drawing on it and using the resulting texture, the texture is drawn as totally black.

If I just don't wrap the drawing code in render_tex->begin(); and render_tex->end();, everything draws correctly, leading me to believe that the problem is isolated to the code above.

Answer 1

Make sure the texture is not bound before trying to render into it. Even if not using texturing at all, trying to render into a currently bound texture may invoke undefined behaviour and just not work.

You should actually call glBindTexture(GL_TEXTURE_2D, 0) after the glTexImage2D in your RenderTexture constructor, or maybe restore the previously bound texture, like you do with the FBO. Just make sure the tex is not bound when you render into the FBO.