OpenGL question

[^NyAw^]

Hi,

I'm new to OpenGL.
I have an image loaded from a JPEG file into a char array(8 bit depth).
I'm using this pice of code to paint a texture on a rectangle just to show the image:

glTexImage2D(GL_TEXTURE_2D,0,1,iWidth,iHeight,0,GL_LUMINANCE,GL_UNSIGNED_BYTE,pcData);
    glBegin(GL_QUADS);
        //glTexCoord2f(0.0f, 1.0f);
        glTexCoord2f(0.0f, 0.0f);
        glVertex2f(0.0f, 0.0f);
 
        //glTexCoord2f(1.0f, 1.0f);
        glTexCoord2f(1.0f, 0.0f);
        glVertex2f(float(width()), 0.0f);
 
        //glTexCoord2f(1.0f, 0.0f);
        glTexCoord2f(1.0f, 1.0f);
        glVertex2f(float(width()), float(height()));
 
        //glTexCoord2f(0.0f, 0.0f);
        glTexCoord2f(0.0f, 1.0f);
        glVertex2f(0.0f, float(height()));
    glEnd();
    glFlush();

glTexImage2D(GL_TEXTURE_2D,0,1,iWidth,iHeight,0,GL_LUMINANCE,GL_UNSIGNED_BYTE,pcData);
    glBegin(GL_QUADS);
        //glTexCoord2f(0.0f, 1.0f);
        glTexCoord2f(0.0f, 0.0f);
        glVertex2f(0.0f, 0.0f);

        //glTexCoord2f(1.0f, 1.0f);
        glTexCoord2f(1.0f, 0.0f);
        glVertex2f(float(width()), 0.0f);

        //glTexCoord2f(1.0f, 0.0f);
        glTexCoord2f(1.0f, 1.0f);
        glVertex2f(float(width()), float(height()));

        //glTexCoord2f(0.0f, 0.0f);
        glTexCoord2f(0.0f, 1.0f);
        glVertex2f(0.0f, float(height()));
    glEnd();
    glFlush();

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If I use the uncommented "glTexCoord2f" functions, the image that I can see is inverted. I think that JPEG files store every line inverted(don't really know why but I think bmps also do this).
If I use the commented "glTexCoord2f" functions, the image is well shown but really don't understand why because I don't understand how "glTexCoord2f" works.

Can anyone tell me this simple question? Thanks,

[oc2k1]

You mean that the texture is mirrored? That is a well known issue, there are some possibilities to solve that problem:

* Mirror the loaded image line by line
* Use the texture matrix (Scale Y with -1.0 and translate by 1.0)
* Use a vertex shader (same effect like the texture matrix)

Many picture formats are encoded from top to bottom, in OpenGL it starts at the Coordinates 0;0 (bottom left)

[^NyAw^]

Hi,

I'm saying that using the "glTexCoord2d" with the same values of the "glVertex2f", the image is mirrored. Then I changed the "glTexCoord2d" to the other ones that are uncommented and it works ok, but I don't understand why because I'm new to OpenGl and don't understand how does it work.

Thanks,