How to Achieve Pixelated Lines When Scaling in OpenGL?

Scaling Single Pixel Lines in OpenGL

When rendering retro 2D pixel graphics, it's common to use a lower internal resolution for pixelation and scale the output to a larger resolution for display. However, scaling GL_LINE_LOOP primitives can result in non-pixelated lines.

Understanding the Issue

OpenGL's glOrtho function defines a 2D coordinate system where the origin is in the bottom-left corner and extends to the top-right. By calling glOrtho(0, 320, 240, 0, 0, 1), you define a 320x240 virtual canvas.

However, when the glViewport function is used to scale the output, the virtual canvas is not physically resized. Instead, the primitives are drawn on the full output resolution (e.g., 960x720) and then scaled up. This results in pixelated rectangles but non-pixelated lines.

Correct Approach

The recommended solution is to render to a texture of the desired virtual resolution (320x240 in this case) and then draw that texture to the window at the desired output resolution (960x720). This ensures that the primitives are rendered at the virtual resolution, maintaining their pixelated appearance.

Steps to Implement

1. Create a texture with a size of 320x240.
2. Attach the texture to a framebuffer object (FBO).
3. Bind the FBO for rendering.
4. Set the viewport to the size of the texture (320x240).
5. Render the primitives to the texture.
6. Unbind the FBO.
7. Set the viewport to the size of the output window (960x720).
8. Bind the texture as a texture unit.
9. Draw the texture to the output window using the appropriate coordinates.

This approach ensures that the lines are drawn at the virtual resolution, maintaining their pixelated appearance even when scaled up to a higher output resolution.

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