One difficulty is that the line rasterization rules for OpenGL are designed to avoid multiple pixel hits when drawing connected line primitives. The reason for this is that multiple hits cause difficulties in using blending or stenciling algorithms to merge multiple primitives reliably. This means that if a rectangle is drawn with a GL_LINE_LOOP, the rectangle will be properly closed with no missing pixels, whereas if the same rectangle is drawn with a GL_LINE_STRIP, or independent GL_LINES there will likely be pixels missing and or multiple hits on the rectangle boundary at or near the vertices of the rectangle. A second issue is that OpenGL does use half integer pixel centers, whereas the native window system specifies pixel centers at integer boundaries. Application developers often incorrectly use integer pixel centers with OpenGL without compensating in the projection transform.