Multiple Viewport OpenGL ES - ios

Question

I am currently trying to make an ios app that uses multiple opengl es viewports (i.e. a split screen). Here's how I'm doing it:
// Create default framebuffer object. The backing will be allocated for the current layer in -resizeFromLayer
glGenFramebuffersOES(1, &defaultFramebuffer);
glGenRenderbuffersOES(1, &colorRenderbuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, defaultFramebuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, colorRenderbuffer);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, colorRenderbuffer);
// Replace the implementation of this method to do your own custom drawing
const GLfloat squareVertices[] = {
-0.5f, -0.5f,
0.5f, -0.5f,
-0.5f, 0.5f,
0.5f, 0.5f,
};
const GLubyte squareColors[] = {
255, 255, 0, 255,
0, 255, 255, 255,
0, 0, 0, 0,
255, 0, 255, 255,
};
[EAGLContext setCurrentContext:context];
glBindFramebufferOES(GL_FRAMEBUFFER_OES, defaultFramebuffer);
glViewport(0, backingHeight/2, backingWidth/2, backingHeight/2);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrthof(-1.0f, 1.0f, -1.5f, 1.5f, -1.0f, 1.0f);
glMatrixMode(GL_MODELVIEW);
glRotatef(3.0f, 0.0f, 0.0f, 1.0f);
glClearColor(0.5f, 0.5f, 0.5f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);
glVertexPointer(2, GL_FLOAT, 0, squareVertices);
glEnableClientState(GL_VERTEX_ARRAY);
glColorPointer(4, GL_UNSIGNED_BYTE, 0, squareColors);
glEnableClientState(GL_COLOR_ARRAY);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, colorRenderbuffer);
// equivalent to glutswapBuffers()
[context presentRenderbuffer:GL_RENDERBUFFER_OES];
/*********************SECOND VIEW*******/
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, defaultFramebuffer);
glViewport(backingWidth/2, 0, backingWidth/2, backingHeight/2);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrthof(-1.0f, 1.0f, -1.5f, 1.5f, -1.0f, 1.0f);
glMatrixMode(GL_MODELVIEW);
glRotatef(3.0f, 0.0f, 0.0f, 1.0f);
glClearColor(0.5f, 0.5f, 0.5f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);
glVertexPointer(2, GL_FLOAT, 0, squareVertices);
glEnableClientState(GL_VERTEX_ARRAY);
glColorPointer(4, GL_UNSIGNED_BYTE, 0, squareColors);
glEnableClientState(GL_COLOR_ARRAY);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, colorRenderbuffer);
[context presentRenderbuffer:GL_RENDERBUFFER_OES];
/*********************END***************/
glDisableClientState(GL_VERTEX_ARRAY);
glDisableClientState(GL_COLOR_ARRAY);
On the simulator, only the second viewport appears (in this case a rotating cube in the bottom right corner of screen). Whereas on the device they both appear but blinking... (top left AND bottom riht)
the glRotatef gets applied twice, but I only want to or any other transformation to be applied once.
What i want is simply to show the same scene twice. Basically I'm trying to simulate having 2 cameras watching the same thing, and show this on the screen. Clearly, I'm doing something wrong, would it be better to have 2 separate framebuffers and renderbuffers or is there a simpler way of achieving this?

Answer 1

The problem is this line:
/*********************SECOND VIEW*******/
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_FRAMEBUFFER);
glClear is not affected by the viewport, you need to use either the scissors test or the stencil buffer if you want to partially clear.
See the documentation for glClear.