Re: [GM-help] How to convert DPX from RGB to grayscale but in log?

[Bob F. <bfr...@si...>]

On Mon, 9 Sep 2013, Joakim Ziegler wrote:
>
>> Due to the problem with writing/reading gray 10-bit DPX, I took steps
>> to avoid writing it accidentally.  Perhaps it is not even possible to
>> trigger writing it with the current code without changing the
>> colorspace to gray.
>
> Is there a way to interpret the input colorspace as linear, though?
> According to the documentation, it seems -define dpx:colorspace=rgb
> should do that, and it indeed changes the contrast, but it's still not
> the same as the log file.

Hopefully you are at least using a Q16 build of GraphicsMagick.  You 
can tell that by the first line of output from 'gm convert -version'. 
A Q8 build will pretend to work fine (even with 10-bit DPX) but lose a 
lot of quality in the levels.

> I don't really care about colorspaces as such, I just want an output
> file that has the same transfer curve as the input, but in grayscale,
> and being one third of the size of the original (and I'd also like to be
> able to convert the other way).

Understood.

In hindsight, the automatic colorspace conversion in GraphicsMagick 
was a mistake.  It would have been better to make conversion to/from 
log/linear an explicit operation.

Bob
-- 
Bob Friesenhahn
bfr...@si..., http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/