Re: Simple API and associated alpha

I've taken another look at the code, it seems like the simplest way to fix
this would be to add PNG_FORMAT_FLAG_ASSOCIATED_ALPHA - would a PR be
accepted?

On 6 September 2017 at 14:13, Samuel Williams <
[email protected]> wrote:

> Okay I took another look at the documentation and I think I see what's
> going on.
>
>
> The channels are encoded in one of two ways:
>   a) As a small integer, value 0..255, contained in a single byte.  For the
> alpha channel the original value is simply value/255.  For the color or
> luminance channels the value is encoded according to the sRGB specification
> and matches the 8-bit format expected by typical display devices.
> The color/gray channels are not scaled (pre-multiplied) by the alpha
> channel and are suitable for passing to color management software.
>   b) As a value in the range 0..65535, contained in a 2-byte integer, in
> the native byte order of the platform on which the application is running.
> All channels can be converted to the original value by dividing by 65535;
> all
> channels are linear.  Color channels use the RGB encoding (RGB end-points)
> of
> the sRGB specification.  This encoding is identified by the
> PNG_FORMAT_FLAG_LINEAR flag below.
> When the simplified API needs to convert between sRGB and linear
> colorspaces,
> the actual sRGB transfer curve defined in the sRGB specification (see the
> article at http://en.wikipedia.org/wiki/SRGB) is used, not the gamma=1/2.2
> approximation used elsewhere in libpng.
> When an alpha channel is present it is expected to denote pixel coverage
> of the color or luminance channels and is returned as an associated alpha
> channel: the color/gray channels are scaled (pre-multiplied) by the alpha
> value.
> The samples are either contained directly in the image data, between 1 and
> 8
> bytes per pixel according to the encoding, or are held in a color-map
> indexed
> by bytes in the image data.  In the case of a color-map the color-map
> entries
> are individual samples, encoded as above, and the image data has one byte
> per
> pixel to select the relevant sample from the color-map.​
>
>
> So, it wasn't clear to me the paragraphs following a) and b) were related.
> So, for part a), this means for single byte pixel formats, "The color/gray
> channels are not scaled (pre-multiplied) by the alpha
> channel and are suitable for passing to color management software." I
> wrongfully inferred the organisation of that documentation due to the lack
> of indentation/titles.
>
> So, I'm going to assume that when loading 8-bit formats, I can't use the
> simple API if I want pre-multiplied data. Is that correct?
>
> Thanks
> Samuel
>

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