[Public WebGL] y-orientation for texImage2D from HTML elements

Florian Bösch [email protected]
Fri Dec 7 07:59:25 PST 2012


On Fri, Dec 7, 2012 at 4:31 PM, Georg Kolling <[email protected]>wrote:

> ****
>
> In my opinion the fault here lies with the GL specification which
> incorrectly applies a physical orientation to an abstract coordinate space.
> ****
>
> ** **
>
> In reality TexImage2D accepts pixel data in “origin-first, S-major
> (S-increasing, T-increasing)” order. The first pixel passed, at the memory
> address given, is taken to be at (or near) the texture space origin. Each
> row of pixels form a line along the positive S axis, and subsequent rows
> are located at increasing T. ****
>
> Physical orientation has no meaning in an abstract coordinate space such
> as texture space. Positive T direction is neither “up” nor “down” (or
> left-right, front-back, north-south, you get the idea).****
>
> If you take the description above, i.e. “origin-first” it immediately
> becomes clear that if you want to preserve the origin location, HTML
> elements with their origin in the top-left corner should be passed to
> TexImage2D top-left corner first.****
>
> Regards,****
>
> Georg
>
When you render to a texture the origin is bottm/left not top/left. If
you're serious about origin preservation such as not to end up with cases
where there is no right UV set anymore or you render things upside down
variably, you upload with origin bottom/left not top/left. Alternatively
you fix glViewport, glScissors, FBOs, to have origin top/left such that
you'll not have to render things upside down and can apply one set of UV
coords regardless.
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