How to Maintain Pixelated Lines When Scaling Graphics in OpenGL?

Scaling and Drawing Lines in OpenGL

In the pursuit of creating 2D pixel art games, you may encounter issues with scaling and pixel size when rendering to a window with a higher resolution. This article delves into a common problem and provides a solution for maintaining pixelated graphics.

The approach described is to establish an internal resolution using glOrtho() while scaling up the output resolution for display on the screen. For instance, an internal resolution of 320x240 could be scaled to 960x720 on the screen.

However, when drawing lines with GL_LINE_LOOP, you may notice that the lines appear thin and pixelated rather than preserving the original pixel size. This occurs because the line is being drawn on the full 960x720 canvas, resulting in a 1px line width in a world of 3px pixels.

To resolve this issue, it is crucial to understand that there is no separate "320x240 glOrtho canvas" within the window's actual resolution. OpenGL scales all drawn primitives based on the window's resolution, including line end points. However, the rasterization of the line between the end points remains based on the actual rendering resolution, leading to non-pixelated diagonal lines.

The proper solution involves rendering to a genuinely 320x240 image and then drawing that image to the window at its higher resolution. This requires creating a 320x240 texture, attaching it to a framebuffer object (FBO), and rendering to it with a viewport set to the image's size. Once the image is rendered to the texture, the FBO can be unbound, and the texture can be drawn to the window with the viewport set to the window's resolution.

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