How can I programmatically render fullscreen openGL at a specific resolution? - macos

Question

I am working on a OSX/Cocoa graphics application which (for performance reasons) I would like to render at 640x480 when the user selects "full screen" mode. For what it's worth, the content is a custom NSView which draws using openGL.
I understand that rather than actually change the user's resolution, it's preferable to change the backbuffer (as explained on another SO question here: Programmatically change resolution OS X).
Following that advice, I end up with the following two methods (see below) to toggle between fullscreen and windowed. The trouble is that when I go fullscreen, the content does indeed render at 640x480 but is not scaled (IE it appears as if we stayed at the window's resolution and "zoomed" into a 640x480 corner of the render).
I'm probably missing something obvious here - I suppose I could translate the render according to the actual screen resolution to "center" it, but that seems overcomplicated?
- (void)goFullscreen{
// Bounce if we're already fullscreen
if(_isFullscreen){return;}
// Save original size and position
NSRect frame = [self.window.contentView frame];
original_size = frame.size;
original_position = frame.origin;
NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithBool:NO],NSFullScreenModeAllScreens,
nil];
// In lieu of changing resolution, we set the backbuffer to 640x480
GLint dim[2] = {640, 480};
CGLSetParameter([[self openGLContext] CGLContextObj], kCGLCPSurfaceBackingSize, dim);
CGLEnable ([[self openGLContext] CGLContextObj], kCGLCESurfaceBackingSize);
// Go fullscreen!
[self enterFullScreenMode:[NSScreen mainScreen] withOptions:options];
_isFullscreen=true;
}
- (void)goWindowed{
// Bounce if we're already windowed
if(!_isFullscreen){return;}
// Reset backbuffer
GLint dim[2] = {original_size.width, original_size.height};
CGLSetParameter([[self openGLContext] CGLContextObj], kCGLCPSurfaceBackingSize, dim);
CGLEnable ([[self openGLContext] CGLContextObj], kCGLCESurfaceBackingSize);
// Go windowed!
[self exitFullScreenModeWithOptions:nil];
[self.window makeFirstResponder:self];
_isFullscreen=false;
}
Update
Here's now to do something similar to datenwolf's suggestion below, but not using openGL (useful for non-gl content).
// Render into a specific size
renderDimensions = NSMakeSize(640, 480);
NSImage *drawIntoImage = [[NSImage alloc] initWithSize:renderDimensions];
[drawIntoImage lockFocus];
[self drawViewOfSize:renderDimensions];
[drawIntoImage unlockFocus];
[self syphonSendImage:drawIntoImage];
// Resize to fit preview area and draw
NSSize newSize = NSMakeSize(self.frame.size.width, self.frame.size.height);
[drawIntoImage setSize: newSize];
[[NSColor blackColor] set];
[self lockFocus];
[NSBezierPath fillRect:self.frame];
[drawIntoImage drawAtPoint:NSZeroPoint fromRect:self.frame operation:NSCompositeCopy fraction:1];
[self unlockFocus];

Answer 1

Use a FBO with a texture of the desired target resolution attached and render to that FBO/texture in said resolution. Then switch to the main framebuffer and draw a full screen quad using the texture rendered to just before. Use whatever magnification filter you like best. If you want to bring out the big guns you could implement a Lancosz / sinc interpolator in the fragment shader to upscaling the intermediary texture.