Texture not displaying properly - probably coordinates are wrong OpenGL, C++

Question

I'll try to explain my problem with images. So this is a test texture I'm using for my OpenGL application:

As you can see, there is a 2-pixels wide border around the image with different colors for me to be able to see if coordinates are properly set in my application.

I'm using a 9-cell pattern so I'm drawing 9 quads with specific texture coordinates. At first sight everything works fine, but there is a small problem with displaying a texture:

In the picture I marked where is first quad, and where is the second one. As you can see, first one is displayed correctly, but second one smoothly goes from first quad's colors to it's own, but it should start with pure green and pink. So I'm guessing it's a problem with texture coordinates.

This is how they are set:

// Bottom left quad [1st quad]
glBegin(GL_QUADS);
    // Bottom left
    glTexCoord2f(0.0f, 1.0);                
    glVertex2i(pos.x, pos.y + height);

    // Top left
    glTexCoord2f(0.0f, (GLfloat)1.0 - maxTexCoordBorderY);  
    glVertex2i(pos.x, pos.y + height - m_borderWidth);

    // Top right
    glTexCoord2f(maxTexCoordBorderX, (GLfloat)1.0 - maxTexCoordBorderY);
    glVertex2i(pos.x + m_borderWidth, pos.y + height - m_borderWidth);

    // Bottom right
    glTexCoord2f(maxTexCoordBorderX, 1.0);
    glVertex2i(pos.x + m_borderWidth, pos.y + height);
glEnd();

// Bottom middle quad [2nd quad]
glBegin(GL_QUADS);
    // Bottom left
    glTexCoord2f(maxTexCoordBorderX, 1.0);          
    glVertex2i(pos.x + m_borderWidth, pos.y + height);

    // Top left
    glTexCoord2f(maxTexCoordBorderX, (GLfloat)1.0 - maxTexCoordBorderY);
    glVertex2i(pos.x + m_borderWidth, pos.y + height - m_borderWidth);

    // Top right
    glTexCoord2f((GLfloat)1.0 - maxTexCoordBorderX, (GLfloat)1.0 - maxTexCoordBorderY);
    glVertex2i(pos.x + width - m_borderWidth, pos.y + height - m_borderWidth);

    // Bottom right
    glTexCoord2f((GLfloat)1.0 - maxTexCoordBorderX, 1.0);   
    glVertex2i(pos.x + width - m_borderWidth, pos.y + height);
glEnd();

You can see that I'm using maxTexCoordBorderX variable which is calculated based on border and image size. Image width is 32 and border width is 2.

maxTexCoordBorderX = 2 / 32 = 0.0625

Could anyone please help with with finding out where the problem is?

Answer 1

Most likely culprit is that you are not sampling on the texel centers. For example, if you have a 32x32 pixel texture, the texel-centers are offset by 1/64.

Here's a rough diagram of a 4x4 texture. The squares are the texels (or pixels) of the image.

_________________1,1
|   |   |   |   |
|   |   |   |   |
|___|___|___|___|_0.75
|   |   |   |   |
|   |   |   |   |
|___|___|___|___|_0.5
|   |   |   |   |
|   |   |   |   |
|___|___|___|___|_0.25
|   |   |   |   |
| X |   |   |   |
|___|___|___|___|
0,0 |  0.5  |   1
   0.25    0.75

x = (0.125, 0.125)

If you sample on one of the lines, you will get a value exactly between two texels, which will (if you have texture sampling set to linear blend) give you an average value. If you want to sample the exact texel value, you need to specify a u,v in the center of the texel.

Answer 2

You've running into a fencepost problem. I answered the solution to your very problem here

https://stackoverflow.com/a/5879551/524368