OpenGL ES Texture Coordinates Slightly Off

Question

I'm trying to draw a subregion of a texture in OpenGL by specifying the coordinates I want. What's happening though is that, depending on the size of the image, it seems there's a slight offset in the origin of where it selects the texture coordinates. The offset amount seems to be less than the size of a pixel & the output is is blurred combination of neighboring pixels.

Here's an idea of what I'm describing. In this case I'd want to select the 6x5 green/white region but what OpenGL is rendering includes a slight pink tint to the top & left pixels.

What the output would look like:

I can fix it by adding an offset to the texture coordinates before passing them to glTexCoordPointer but the problem is that I have no way to calculate what the offset is and it seems different for different textures.

Pseudocode:

float uFactor = regionWidth / textureWidth;   // For the example: 0.6f
float vFactor = regionHeight / textureHeight; // For the example: 0.5f

data[0].t[0] = 0.0f * uFactor;
data[0].t[1] = 0.0f * vFactor;
data[1].t[0] = 1.0f * uFactor;
data[1].t[1] = 0.0f * vFactor;
data[2].t[0] = 0.0f * uFactor;
data[2].t[1] = 1.0f * vFactor;
data[3].t[0] = 1.0f * uFactor;
data[3].t[1] = 1.0f * vFactor;

glPushMatrix();

// translate/scale/bind operations

glTexCoordPointer(2, GL_FLOAT, 0, data[0].t);

Answer 1

Keep in mind, that OpenGL samples textures at the texel centers. So when using linear filtering (like GL_LINEAR or GL_LINEAR_MIPMAP_LINEAR) the exact texel color is only returned if sampled at the texel center. Thus, when you only want to use a sub-region of a texture, you need to indent your texture coordinates by half a texel (or 0.5/width and 0.5/height). Otherwise the filtering will blend the border of the texture with neigbouting texels outside of your intended region. This causes your slightly pinkish border. If you use up the whole texture this effect is compensated by the GL_CLAMP_TO_EDGE wrapping mode, but when using a subregion, GL does not know where it's edge is and that filtering should not cross it.

So when you got a subregion of the texture in the range [s1,s2]x[t1,t2] (0 <= s,t <= 1), the real valid texCoord interval should be [s1+x,s2-x]x[t1+y,t2-y] with x being 0.5/width and y being 0.5/height (the width and height of the whole texture corresponding to [0,1]x[0,1]).

Therefore try

data[0].t[0] = 0.0f * uFactor + 0.5/textureWidth;
data[0].t[1] = 0.0f * vFactor + 0.5/textureHeight;
data[1].t[0] = 1.0f * uFactor - 0.5/textureWidth;
data[1].t[1] = 0.0f * vFactor + 0.5/textureHeight;
data[2].t[0] = 0.0f * uFactor + 0.5/textureWidth;
data[2].t[1] = 1.0f * vFactor - 0.5/textureHeight;
data[3].t[0] = 1.0f * uFactor - 0.5/textureWidth;
data[3].t[1] = 1.0f * vFactor - 0.5/textureHeight;

Answer 2

Propably this has to do with the wrap around. Try this when you create the source image:

glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP )
glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP )