Re: [Openexr-devel] Invert lines during reading process ?

[Florian Kainz]

Hi Gernot,

in order to load an image "upside down" you'd have to set
your frame buffer's yStride to a negative value, but since
yStride is of type size_t, which is unsigned, you can't
do that.

However, there is a workaround: unsigned integer arithmetic
is usually implemented such that results are computed modulo
(1<<b), where b is the number of bits in an unsigned integer.
If n is positive and of type size_t, you can effectively set
yStride to -n like this:

    yStride = (~(size_t(0)) - n + 1);

If x is of type size_t, and x >= n, then computing x + yStride
yields x - n.

Florian


Gernot Ziegler wrote:
> Hej !
>
> OpenGL convention requires me to upload images upside down into the
> texture data buffer, that is, I need to read the EXR image from the bottom
> up - is there a way to do this more efficiently than to read the lines one
> by one (the headers lineorder attribute is only changeable for writing
> files, correct ?) ?
>
> If only the line-by-line reading remains:
> Should I maybe y-flip the data in main memory instead ? (Main memory is
> not an issue :-) )
>
> Servus,
>   Gernot