OpenGL 1.1 render at fixed resolution and upscale-CodePudding

I'm trying to render a scene at 320x240 permanently to emulate the feel classic 240p systems, and upscale it to the full size of the screen.

The issue I have is that, although I can sort of fudge this effect using sprites of a set resolution and glOrtho, it often just renders completely at the higher resolution and ends up being maybe a little slower, and rotating a sprite would end up making it obvious that the resolution is a lot higher.

Is there some way to render the viewport at 320x240, and then upscale it to fit the screen, using OpenGL 1.1?

Right now I'm using this to set up the 2D viewport for rendering, where width and height are the width and height of the canvas.

GL11.glViewport(0, 0, width, height);
GL11.glDepthFunc(GL11.GL_LEQUAL);
GL11.glClear(GL11.GL_ACCUM);
GL11.glMatrixMode(GL11.GL_PROJECTION);
GL11.glLoadIdentity();
GL11.glOrtho(0.0D, 320, 240, 0.0D, 0.0D, 100.0D); //320 and 240 is the "true size"
GL11.glMatrixMode(GL11.GL_MODELVIEW);
GL11.glLoadIdentity();

I've heard about rendering onto textures, however I cannot find any information on how to do this in OpenGL 1.1.

CodePudding user response：

I've heard about rendering onto textures, however I cannot find any information on how to do this in OpenGL 1.1.

That's because OpenGL 1.1 is from 1997 - literally a quarter century ago. There's no render-to-texture there.

The closest thing you get is glCopyTexImage2D, which allows you to copy framebuffer contents into a texture object (which technically would allow this to happen on the GPU side without a round-trip to CPU and system memory). So you basically render your data in the internal resolution - using only a portion of the framebuffer as the viewport, copy that into a texture, and then draw a rectangle on the full framebuffer with that texture and the desired filters.

But be warned that GL_MAX_TEXTURE_SIZE is guaranteed to be at least 64 in GL 1.1, so to write this in a way thatb it would work on any conformant GL 1.1 inmplementation, you would have to have code to split your framebuffer texture into 64x64 pixel tiles.

The other alternative - a roundtrip to system memory - would involve reading the data back with glReadPixels, and drawing it via again glDrawPixels while applying the glPixelZoom setting.