Wrong colors when loading textures in OpenGL - c++

Question

I'm trying to understand how to load a texture in OpenGL and I wrote this very simple code:
GLuint texture;
void loadTexture() {
GLubyte data[] = { 255,0,0,
0,255,0,
0,255,0,
255,0,0 };
glGenTextures(1, &texture);
glBindTexture(GL_TEXTURE_2D, texture);
glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT );
glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT );
int chk = gluBuild2DMipmaps( GL_TEXTURE_2D, GL_RGB, 2, 2, GL_RGB, GL_UNSIGNED_BYTE, data );
if (chk!=0)
printf("error code = %d\n",chk);
else
printf("success\n");
}
I use loadTexture() to load the texture in memory. The texture, in this sample, is extremely simple, but at the moment it doesn't matter.
void drawTexturedSquare() {
glEnable( GL_TEXTURE_2D );
glBegin (GL_QUADS);
glTexCoord2f (0.0, 0.0);
glNormal3f(0, 0, 1);
glVertex3f (0.0, 0.0, 0.0);
glTexCoord2f (1.0, 0.0);
glNormal3f(0, 0, 1);
glVertex3f (10.0, 0.0, 0.0);
glTexCoord2f (1.0, 1.0);
glNormal3f(0, 0, 1);
glVertex3f (10, 10, 0.0);
glTexCoord2f (0.0, 1.0);
glNormal3f(0, 0, 1);
glVertex3f (0.0, 10, 0.0);
glEnd ();
glDisable( GL_TEXTURE_2D);
}
I would like to apply this simple texture to a square. I call function drawTexturedSquare() from inside a draw() function where I already called
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
The problem is that this is the result I get
while I expected the square to be green in main diagonal (upper-left to lower-right) and red is secondary diagonal (upper-right to lower-left). May someone explain me why?
Besides, every time I run the program I get a different result:
I do not understand where this blue comes out...May someone help me?

Answer 1

Each row of your data needs to be 4-byte aligned. Either pad each row with 0 0, or use a RGBA texture.
If you don't want to do the aforementioned, you can use:
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
The reason your color is changing is that since you don't pad your rows correctly, OpenGL reads past the end of the array.

Answer 2

You may want to try GL_NEAREST for GL_TEXTURE_MIN_FILTER and GL_TEXTURE_MAG_FILTER
As it is right now, OpenGL is interpolating between the colors, creating a gradient (since your texture is being stretched from 2x2 to however big your screen is)
This doesn't explain why you're getting different results for your texture each time though.