Re: Move rgb2ntsc and ntsc2rgb to image package

[Carnë Draug]

On 17 January 2018 at 05:25, Rik <address@hidden> wrote:
> On 01/16/2018 10:04 AM, Carnë Draug wrote:
>> On 16 January 2018 at 16:06, Rik <address@hidden> wrote:
>>> Carnë,
>>>
>>> As you are maintainer of the image package, I'm writing to coordinate on
>>> some of of the lesser used image functions in core Octave that should move
>>> to the image package.  This sort of clean-up has already been done for the
>>> statistics functions.
>>>
>>> It seems like rgb2ntsc and ntsc2rgb are pretty specific and should be moved
>>> to the image package.  However, the general functions rgb2gray and gray2rgb
>>> which use the luminance channel of rgb2ntsc should be written.  I don't
>>> mind doing that if you can take care of incorporating the other two into
>>> the image package.
>>>
>>> Cheers,
>>> Rik
>> ok. I think there's more functions like ind2gray and gray2ind so let
>> me know.  I have a patch ready for the functions you mentioned.
>>
>> Carnë
>
> I added rgb2gray.m here
> (http://hg.savannah.gnu.org/hgweb/octave/rev/3ad53e4793fc).  I removed
> rgb2ntsc.m and ntsc2rgb.m here
> (http://hg.savannah.gnu.org/hgweb/octave/rev/afbef2f579c9).  It should be
> okay to push your patch for the image package now.

Unlike the version of rgb2ntsc, rgb2gray should return the same data
type as the input image.  On the image package, that happens with the
imcast/im2X functions but they are not in core [1].  Not sure how to
best handle this in core without having those functions duplicated.

Also, despite what Matlab says on the documentation, they don't seem
to use that transformation matrix with only 3 decimal cases:

    >> T = rgb2gray ([1 0 0; 0 1 0; 0 0 1]);
    >> T(:,1)
    ans =
       0.298936021293776
       0.587043074451121
       0.114020904255103

so might be better to had a few more decimal cases.  At least so it's
compatible with the existing function in the image package.

Finally, the reshaping back to the original shape can be done with the
existing private function colorspace_conversion_revert [2].

Carnë

[1] http://hg.code.sf.net/p/octave/image/file/b2a85fb8b517/inst/rgb2gray.m#l50
[2] 
http://hg.savannah.gnu.org/hgweb/octave/file/afbef2f579c9/scripts/image/private/colorspace_conversion_revert.m