Using Subtextures

To load a subrectangle of a texture image, call glTexSubImage1DEXT(), glTexSubImage2DEXT()--prototype below--or glTexSubImage3DEXT().

void glTexSubImage2DEXT(enum target, int level, int xoffset,int yoffset,

sizei width, sizei height, enum format, enum type, const void* pixels)

target specifies the existing texture image for which the subimage is being defined.
level, depth, format, type, and pixels correspond to the arguments of glTexImage2D(); width and height are those of the subimage.
xoffset and yoffset specify texel offsets in the x and y directions within the texture image being modified.

Using any uninitialized portion of a texture while drawing yields undefined results.