Mimic the letterbox behaviour of the Qt viewfinder by rendering the image centered. This is done by adding a projection matrix to the vertex shader to scale the rendered rectangle. Another option would have been to keep using glViewport() (which would have needed to be moved to paintGL(), as Qt resets the viewport to span the full widget before calling). Hidpi displays would then need special handling of the device pixel ratio, which is done automatically by Qt when it sets the default viewport. Using a projection matrix avoids this complication. Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Tested-by: Kieran Bingham <kieran.bingham@ideasonboard.com> Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>