If this flag is set, the texture drawing functions will use the OpenGL
TEXTURE_MATRIX instead of the MODELVIEW_MATRIX for application of a
rotation transform - ie. for rotated drawing of textures.
Conceptually you can think of the difference as follows:
Normally, you pass a rectangular, upright ‘dstRect’ destination rectangle
that defines the area of the window that should be overdrawn by the
texture. If the ‘rotationAngle’ is non-zero, this ‘dstRect’ will be
rotated around its center, thereby drawing a rotated texture onto the
screen. The pixel data is read from the upright ‘srcRect’ source
rectangle in the texture image matrix.
If you set this flag, then the destination rectangle ‘dstRect’ stays at
its defined upright position, whereas the ‘srcRect’ rectangular source
area is rotated by ‘rotationAngle’ around its center, so a rotated area
of the texture image matrix is read out, but drawn upright into the
window. The advantage of this mode is that you can draw a lot rotated
textures tightly packed/close to other parts of your stimulus without
If you use the 2nd mode of drawing, you’ll need to make sure that the
‘srcRect’ source rectangle only occupies a fraction of the texture image
and that you’ll have at least d * sqrt(2) pixels space of defined texture
data around the center of the ‘srcRect’, when d is the maximum of the
width and height of the rectangle. Otherwise the “rotated reading” from
the texture would actually try to read from areas outside the texture
matrix - this will create undefined visual results, but most likely not
what you want.