OpenGL allows to automatically create mipmaps for you with glGenMipmap.
But you can upload you own mipmaps using glTexImage2D.
for(int lvl = 0; numMips; lvl++) {
glTexImage2D(GL_TEXTURE_2D,
lvl,
GL_RGBA8,
texture[lvl].w, texture[lvl].h, 0,
GL_RGBA, GL_UNSIGNED_BYTE, texture[lvl].data
);
}
However, I wanted to provide my own images only for the upper levels, and let OpenGL compute the rest.
This is possible to do because OpenGL will generate mipmaps starting from GL_TEXTURE_BASE_LEVEL.
So you can do it like this:
// upload custom images
for(int i = 1; i < numCustomMips; i++) {
glTexImage2D(GL_TEXTURE_2D,
lvl,
GL_RGBA8,
texture[lvl].w, texture[lvl].h, 0,
GL_RGBA, GL_UNSIGNED_BYTE, texture[lvl].data
);
}
// set the base level to the smallest mip we provided
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_BASE_LEVEL, numCustomMips-1);
glGenerateMipmap(GL_TEXTURE_2D);
// restore base level
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_BASE_LEVEL, 0);
If you find some textures look tilted, it could be that your texture rows are not 4-byte aligned for higher mips. Try if doing this fixes your problem:
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);