All OpenGL calls are quite expensive if look at the time needed for those calls (see part 5). In game development we want to reduce the amount of time needed to update and render a frame to minimum so we need to create a system that will need as little OpenGL calls as possible. This tutorial will show you how to use texture atlases so that you decrease the render calls quite a bit. This tutorial will continue on the code of the previous tutorials.
In our third shaders tutorial we will use the knowledge we have gained from the second tutorial and we use it to create gray scaled textures. You can create a texture that is already gray scaled and use it with our simple texture shader and you are right, however, there are some situation in which you want to be able to do it runtime. We will show you various forms of gray scaling available using shaders.