How to Draw 1px Lines in OpenGL with Internal Resolution Scaling?

Scaling 1px Lines in OpenGL

Problem:

A game with an internal resolution of 320x240 is scaled to display at higher resolutions (e.g., 960x720). While pixelated graphics are rendered correctly, lines drawn with GL_LINE_LOOP are scaled up to 3px, resulting in incorrect pixel art.

Question:

How can lines be drawn to the internal 320x240 view instead of the 960x720 display view?

Answer:

The misconception is the existence of a "320x240 glOrtho canvas." OpenGL only renders to the actual window resolution (e.g., 960x720). Scaling coordinates merely transforms primitive end points but does not affect rasterization.

To correctly draw 1px lines:

1. Create a 320x240 texture and attach it to a framebuffer object (FBO).
2. Bind the FBO and set the viewport to the texture's size (320x240).
3. Render to the texture using glOrtho(0, 320, 240, 0, 0, 1).
4. Unbind the FBO.
5. Bind the texture and draw it to the window, setting the viewport to the window's size (e.g., 960x720).

By rendering to an offscreen image of the actual resolution and then scaling it up, proper pixelated line rasterization can be achieved.

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