Re: Linear Colorspace

On 12/2/15 2:20 PM, Bob Friesenhahn wrote:
> On Wed, 2 Dec 2015, Sam Wilson wrote:
>
>> Hey graphicsmagick-help,
>>
>> I need your assistance! I'm trying to resize an image
>> (http://www.4p8.com/eric.brasseur/gamma_dalai_lama_gray.jpg) using a
>> linear colorspace. I can't seem to get GraphicsMagick to actually do the
>> conversion. I'm probably making a really dumb mistake, but any input
>> would be hugely appreciated. The code below seems to work with
>> ImageMagick 6.9.2-7, but not with GraphicsMagick 1.3.23 (obviously
>> change MagickCore to MagickLib.)
> One thing I see is use of ColorspaceType::sRGBColorspace.  In
> GraphicsMagick this is a special-purpose colorspace just for Kodak
> PhotoCD and not sRGB.  Use ColorspaceType::RGBColorspace instead.
Ah, this definitely has something to do with it. In recent ImageMagick 
releases, RGB is a linear colorspace, while sRGB is not.
> What do you mean by "Linear Colorspace"?  What are you actually trying
> to accomplish?
I'd like to get a colorspace with 1.0 gamma, I think. If you try to 
scale the image I linked down by 50% without converting the colorspace, 
you'll end up with a grey box. I'd like that not to happen.
> Sometimes "linear" is used to describe gamma coding, but could be
> "linear light" (no gamma) or "linear intensity" (often 1.2 gamma).
> GraphicsMagick assumes that RGB data is similar to "linear intensity"
> sRGB (or Rec.709) but without using ICC color profiles there is no
> assurance of the actual colorspace.
Fairly sure I mean linear light. I'm just getting into the graphics 
world, so please forgive any obvious mistakes on my part. There's an 
article that describes the problem I'm seeing here: 
http://www.4p8.com/eric.brasseur/gamma.html

Thanks for your help,
Sam

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