Re: [Openexr-devel] newbie questions: alpha-only files and linear conversion on save

[Paul Miller]

> 1) Using the RGBA-only interface, one way to write an alpha-only
>    image is to open the file like this:
>
>         RgbaOutputFile file (fileName, width, height, WRITE_A);

Make take on this is I still need to feed it an complete RGBA frame buffer, 
and it only saves the alpha channel. So for maximum performance I would 
prefer the manual header channel insert, since I would only need to 
"convert" my alpha components.

>    OpenEXR expects pixel data to be linear.  In order to convert
>    8-bit pixels to OpenEXR's "half" you should linearize the data:

Ahh - this clears that up a good deal, thanks.

>    Note that this may apply only to RGB data.  In many file formats
>    alpha is linear.  The opacity represented by the alpha channel
>    is proportional to the 8-bit value stored in the pixel.

Yes, I am currently just converting my alpha components to half by dividing 
by 255 (or 65535).

On load, I am currently using the conversion code found in the exrdisplay 
tool.

Thanks for the help guys!