Question

I use the glTranslate command to shift the position of a sprite which I load from a texture in my iPhone OpenGL App. My problem is after I apply glTranslatef, the image appears a little blurred. When I comment that line of code, the image is crystal clear. How can i resolve this issue???

Answer 1

You’re probably not hitting the screen pixel grid exactly. This will cause texture filtering to blur it. The issue is a bit complicated: Instead of seeing the screen an texture as a array of points, see it as sheets of grid ruled paper (the texture sheet can be stretched, sheared, scaled). To make things look crisp the grids must align perfectly. The texture coordinates (0,0) and (1,1) don’t hit the center of the texels but the outer edges of the texture sheet. Thus you need a little bit to offset and scale to address the texel centers. And the same goes for placing the target quads on the screen, where the vertex position must be aligned with the edges of the screen, not the pixel centers. If your projection and modelview matrix are not setup in a way that one unit in modelview space is one pixel wide and the projection fills the whole screen (or window viewport) it’s difficult to get this right.

One normally starts with

glViewport(0,0, width, height);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0, width, 0, height, -1, 1);

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

// modelview XY range 0..width x 0..height now covers the whole viewport
// (0,0) don't address the lower left pixel but the lower left edge of this
// (width,height) similarily addresses the upper right corner

// drawing a 0..width x 0..height quad with texture coordinates 0..1 x 0..1
// will cover it perfectly

This will work as long as the quad as exactly the same dimensions (i.e. it’s vertex positions match) the texture coordinates and the vertex positions are integers.

Now the interesting part is: What if they don’t meet those conditions. Then aliasing occours. In GL_NEAREST filtering mode things still look crisp, but some lines/rows are simply missing. In GL_LINEAR filtering mode neighbouring pixels are interpolated with the interpolation factor beding determined how far off grid they are (in laymans terms, the actual implementation looks slightly different).

So how to solve your issue: Draw sprites in a projection/modelview that matches with the viewport, use only integer coordinates for the vertex coordinates and make your texture cover the whole picture. If you’re using only a part of the texture coordinate range, things get even more interesting, since one addressed the texture grid, not the texel centers.