[HOpenGL] Textures from SDL upside down

[Sebastian Sylvan]

On 3/13/07, Sven Panne <sven.panne at aedion.de> wrote:
> On Tuesday 06 March 2007 00:25, Bas van Dijk wrote:
> > I am using HOpenGL in combination with hsSDL
> > (http://darcs.haskell.org/~lemmih/hsSDL). This works great except for
> > texture loading. All my textures are mirrored over the v-axis. So the
> > top of a texture becomes the bottom. I first thought that I got the
> > texture coordinates wrong. But the textures are also flipped on the GLU
> > quadrics.
>
> IIRC, in SDL the y-coordinate increases when going down the screen, while
> OpenGL uses the usual mathematical convention of y increasing when going up.
> Therefore your images are flipped, I guess.
>
> > Is there some way to tell (H)OpenGL that a texture is mirrored? I tried
> > glPixelStore parameters but I don't think it is possible that way.
> > Perhaps it is possible to change a hsSDL surface before loading the
> > pixel data. Any advice is welcome.
>
> Usually the easiest and most efficient way is to simply flip your t texture
> coordinate, i.e. use 1-t instead of t.

Texture coordinates are not always guarnteed to be in the [0,1] range
though. So a coordinate of 1.2 would become -0.2 which will not always
sample the correct texel (depending on texture wrapping mode).


-- 
Sebastian Sylvan
+46(0)736-818655
UIN: 44640862