Question

I am using Xcode 4.5.1 and testing on an iPhone5 with iOS6.

I was using the frame capture function without problem, but suddenly it stopped working.
When I press the frame capture button, it seems the frame is captured, and the phone switches to a blank screen, only to suddenly switch back to the application screen, and the application keeps running. I can still debug and pause the application, but there’s no way to get the frame capture. I don’t see any errors in the console either.

The reason it stopped working is this piece of code. This code is supposed to render something to a rendertexture, but the rendertexture seems blank. I wanted to use the frame capture function to find out what’s wrong, but the code itself won’t let me capture… 🙁

Any idea why?

// ------------- init function -----------------
// Create the framebuffer and bind it
glGenFramebuffers(1, &g_framebuffer);
glBindFramebuffer(GL_FRAMEBUFFER, g_framebuffer);
//Create the destination texture, and attach it to the framebuffer’s color attachment point.
glGenTextures(1, &g_texture);
glBindTexture(GL_TEXTURE_2D, g_texture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA,  w, h, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, g_texture, 0);    
//Test the framebuffer for completeness
GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER) ;
if(status != GL_FRAMEBUFFER_COMPLETE) {
    NSLog(@"failed to make complete framebuffer object %x", status);
} else {
    NSLog(@"SkyPlugin initialized");
}


// ----------------- on update  ------------------
glGetIntegerv(GL_FRAMEBUFFER_BINDING, &oldFBO);
glGetIntegerv(GL_VIEWPORT, oldViewPort);
// set the framebuffer and clear
glBindTexture(GL_TEXTURE_2D, 0);
glBindFramebuffer(GL_FRAMEBUFFER, g_framebuffer);
glViewport(0, 0, 32, 32);
//glClearColor(0.9f, 0.1f, 0.1f, 1.0f);
glDisable(GL_DEPTH_TEST);
glClear(GL_COLOR_BUFFER_BIT);
// Set shader   
glUseProgram(m_program);
// do some glEnableVertexAttribArray
// ...
// texture setting
glActiveTexture(GL_TEXTURE0);
glUniform1i(m_uniform, 0);
ResourceManager* resourceManager = ResourceManager::GetInstance();
glBindTexture(GL_TEXTURE_2D, m_texture[0]);
// ----------- Draw -----------
// Draws a full-screen quad to copy textures
static const vertexDataUV quad[] = {
    {/*v:*/{-1.f,-1.f,0}, /*t:*/{0,0}},
    {/*v:*/{-1.f,1,0}, /*t:*/{0,1}},
    {/*v:*/{1,-1.f,0}, /*t:*/{1,0}},
    {/*v:*/{1,1,0}, /*t:*/{1,1}}
};
static const GLubyte indeces[] = {0,2,1,3};
glVertexAttribPointer(m_posAttrib, 3, GL_FLOAT, 0, sizeof(vertexDataUV), &quad[0].vertex);
glVertexAttribPointer(m_texCoordAttrib, 2, GL_FLOAT, 0, sizeof(vertexDataUV), &quad[0].uv);
glDrawElements(GL_TRIANGLE_STRIP, 4, GL_UNSIGNED_BYTE, indeces);
// ------------ End
// go back to the main framebuffer!
glBindFramebuffer(GL_FRAMEBUFFER, oldFBO);
glViewport(oldViewPort[0], oldViewPort[1], oldViewPort[2], oldViewPort[3]);
glEnable(GL_DEPTH_TEST);
//glClearColor(0.1f, 0.1f, 0.1f, 1.0f);

Edit: (2012/October/28)

I found out why the above code was not working. I forgot to bind a render buffer! The code below works, but still the frame capture fails when this code is active…

On init,

// Create the renderbuffer and bind it
glGenRenderbuffers(1, &g_renderbuffer);
glBindRenderbuffer(GL_RENDERBUFFER, g_renderbuffer);
glRenderbufferStorage(GL_RENDERBUFFER, GL_RGBA8_OES, w, h);

// Create the framebuffer and bind it
glGenFramebuffers(1, &g_framebuffer);
glBindFramebuffer(GL_FRAMEBUFFER, g_framebuffer);
glFramebufferRenderbuffer(GL_FRAMEBUFFER,
                          GL_COLOR_ATTACHMENT0,
                          GL_RENDERBUFFER, g_renderbuffer);
//Create the destination texture, and attach it to the framebuffer’s color attachment point.
glGenTextures(1, &g_texture);
glBindTexture(GL_TEXTURE_2D, g_texture);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA,  w, h, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, g_texture, 0);

On update,

glGetIntegerv(GL_RENDERBUFFER_BINDING, &oldRBO);
glGetIntegerv(GL_FRAMEBUFFER_BINDING, &oldFBO);
glGetIntegerv(GL_VIEWPORT, oldViewPort);

// set the framebuffer and clear
glBindTexture(GL_TEXTURE_2D, 0);
glBindFramebuffer(GL_FRAMEBUFFER, g_framebuffer);
glBindRenderbuffer(GL_RENDERBUFFER, g_renderbuffer);
glViewport(0, 0, 32, 32);

// ... draw stuff ...

End of update,

// go back to the main framebuffer!
glBindFramebuffer(GL_FRAMEBUFFER, oldFBO);
glBindRenderbuffer(GL_RENDERBUFFER, oldRBO);

Answer 1

It seems it was a bug of Xcode.
The latest version, Xcode 4.5.2 lets me capture the frame 🙂

After I capture the frame, I get an error in this part of the code:

// ... draw stuff ...
glActiveTexture(GL_TEXTURE0);
glUniform1i(MY_TEXTURE, 0);

On the glUniform1i I get this error: “The specified operation is invalid for the current OpenGL state”.

No idea why I get this error (the rendering it’s working), but I suspect this error may be the reason why I wasn’t able to capture a frame in the previous version of Xcode…