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

[Florian Kainz]

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);

   With the general (arbitrary set of channels) interface, you can
   open an alpha-only file like this:

        Header header (width, height);
        header.channels().insert ("A", Channel (HALF));
        OutputFile file (fileName, header);

2) 8-bit (0-255) pixel data are typically not linear in the sense
   that the amount of light represented by the pixel is proportional
   to the number stored in the pixel.  8-bit pixel data are usually
   gamma-corrected; the amount of light represented by  a pixel with
   value v is roughly proportional to pow(v,gamma), where gamma is
   about 2.2.  (For details see Charle's Poynton's Gamma FAQ at
   http://www.poynton.com/GammaFAQ.html.)

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

        half h = pow (v, 1/gamma) / 255.0;

   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.


Paul Miller wrote:
> Hi guys. I'm adding EXR support to our up-coming cross-platform 
> rotoscoping system (Silhouette Roto).
>
> I have two questions:
>
> 1) what is the proper way to set up the header for an "alpha-only" image?
>
> 2) when saving generic linear color data (ie. 0-255), is it sufficient 
> to just normalize to 0-1 and stuff the components into the output half 
> types? This seems to work ok when reading the image back in, as long as 
> I adjust the gamma to 1.0. If I should apply the output gamma 
> correction, can someone post a code snippet?
>
> Thanks!
>
>
>
>
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