I have an FBO with an attached 16bit RGBA color texture target. I bind the fbo, set the draw buffers, clear, and set the viewport equal to the dimensions of the render texture, activate the shader, draw the objects, all works.

Using the same setup, I wanted to render into only the bottom left corner of the FBOs attached target. So I set the viewport to 0, 0, w/2, h/2.
Now I either get nothing, or some sort of partial image only If I dont move the camera after startup.

To do what Im asking, should it be as straight forward as simply changing the viewport, or is there something extra that Im missing when rendering to texture with FBO?