Editing WoW:SetTexCoord Transformations (section)

===Textures and the screen===
For the Texture frame coordinate to Screen coordinate mapping there's a simple translation and scaling process, since we cannot shear or rotate on-screen frames, but for the mapping from image coordinates to the Texture frame, the full set are available.

Logically the process by which a point is transformed from image space to a point on screen can be represented by two transformations T, the transformation from the image to the Texture, and then S, the transformation from the Texture to the screen (I'll go into the math later):

Texture coordinate from image coordinate: t = T * i
Screen coordinate from Texture coordinate: s = S * t

The S transformation is implied by the anchors, size, and scale of the Texture frame element (and the Frame it's contained within).