let’s assume I have a current viewport set (glViewport(…))
with 1200x800 and a texture has the same size. Then I have no
rim at the border of the texture. You can see this at the picture
On the top image u can see there is no rim. the rexture is as large as the current viewport. The bottom image shows a texture that is below the current viewport. Why do I have those rims ? The smaller the texture is, with respect to the current viewport, the greater gets the rim.