Question

I’m using some demos that came with my OpenGL ES emulator as a starting point. The demo I’m currently using is 960×540 pixels with a simple 2D triangle drawn in the middle of the screen.

I see that the triangle has been drawn with floats, and that the lower-left corner of the screen is (-1,-1) while the upper-right corner of the screen is (1,1).

Is it possible to change this? I would like to use integers instead of floats, and have the lower-left corner of the screen be (1,1) while the upper-right corner is (960,540).

This would be much easier to work with because I plan to make a 2D platformer game. I do not want to split any pixels in this game; everything (textures, player movement, camera movement) will conform to whole-pixel coordinates.

I tried crunching the numbers (using the regular grid and floats) to draw a right triangle that was 24×24 pixels. However, it ended up being 24×23 pixels on the screen. So I didn’t want to continue going forward if there was a more accurate, faster method that could use pixel coordinates.

Answer 1

The values coming out of the vertex shader should be ready for projection into the normal OpenGL eye space, but you needn’t pass in vertices in that space. In ES 1 you’d have adjusted the projection stack; does your demo have any analogue to that?

You should also take the filling rules into account. A pixel is always filled if its centre is within the geometry; if its centre is on the boundary then the implementation will make a decision usually depending on which boundary the pixel is on — the top, bottom, left or right boundaries (which make more sense if you think in terms of the pixel output; so a pixel on the left boundary is one that would be first on a scanline, a pixel on the right would be last, a pixel on the top would start the polygon further up the screen, a pixel on the bottom would extend it further down).

The aim is that OpenGL doesn’t draw pixels twice where geometry meets on an edge.

With that in mind, a triangle can be exactly 24 pixel units tall but if it runs from the centre of one pixel to the centre of another it will paint only [at most] 23 rows. So your maths may already be correct. Consider one polygon from y = 0 to y = 24 and one from y = 24 to y = 36 — you don’t want them both painting at y = 24, especially as they may be partially transparent.