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Re: ImageResize does not linearize before resize?

  • To: mathgroup at
  • Subject: [mg117098] Re: ImageResize does not linearize before resize?
  • From: Jari Paljakka <spam at here.invalid>
  • Date: Wed, 9 Mar 2011 07:02:55 -0500 (EST)
  • References: <il52cq$nr6$>

Hi Matthias,

Matthias Odisio <matthias at> wrote:
>On 3/6/11 4:45 AM, Jari Paljakka wrote:
>> Hi,
>> While trialing Mathematica 8, I noticed ImageResize[] doesn't seem to 
>> the colorspace before resizing.
>> Is this intentional?
>> Reasoning why it would be a nice option to have: 
>> Of course, to "fix" this properly, image needs to have information in what 
>> colorspace it is in in the first place, but to have
>> the option available directly in ImageResize would be nice, as I suspect it 
>> could be made quite a bit faster.
>> This non-linearization is probably assumed for all other image functions too?
>> Example how to expose the issue (and how to fix, albeit slow and convoluted):
>> (used "Dalai Lama" pic from the link above, which is constructed to exhibit 
>> issue)
>> (remove compilation target etc if you don't have a compiler - was testing 
>> to make corrections run "faster")
>> Clear[srgb2linear, linear2srgb, linearresize, image]; 
>> srgb2linear = Compile[{{Csrgb, _Real, 1}}, 
>>        With[{\[Alpha] = 0.055}, Table[Piecewise[
>>              {{C/12.92, 
>>         C <= 0.04045}, {((C + \[Alpha])/(1 + \[Alpha]))^2.4, 
>>                  C > 0.04045}}], {C, Csrgb}]], RuntimeAttributes -> 
>>          {Listable}, Parallelization -> True, CompilationTarget -> 
>>          "C", RuntimeOptions -> "Speed"]; 
>> linear2srgb = Compile[{{Clinear, _Real, 1}}, 
>>        With[{\[Alpha] = 0.055}, Table[Piecewise[
>>              {{12.92*C, 
>>         C <= 0.0031308}, {(1 + \[Alpha])*C^(1/2.4) - \[Alpha], 
>>                  C > 0.0031308}}], {C, Clinear}]], 
>>        RuntimeAttributes -> {Listable}, Parallelization -> True, 
>>        CompilationTarget -> "C", RuntimeOptions -> "Speed"]; 
>> linearresize[image_Image, scaling_] := 
>>    Module[{}, ((Image[#1, ColorSpace -> "RGB"] & )[
>>             linear2srgb[ImageData[#1, Interleaving -> True]]] & )[
>>        ImageResize[(Image[#1, ColorSpace -> "RGB"] & )[
>>            srgb2linear[ImageData[image, Interleaving -> True]]], 
>>          scaling]]]
>> image = Image["Dalai Lama", "Real"]
>> In[432]:= ImageResize[ImageResize[image, Scaled[1/2]], Scaled[2]]
>> Out:= Screwed up picture
>> In[433]:= linearresize[linearresize[image, Scaled[1/2]], Scaled[2]]
>> Out:= Correct image (of course sampled)
>This genuine issue will be properly addressed in a future release of
>Mathematica. In the meanwhile, one needs to revert to bits of code,
>as you did.

Thank you very much for the confirmation! I was not sure if it was something 
that would be addressed, or if all imagedata was to be assumed linearized.

>In this particular case, your code produces on my machine a white
>image. That gamma correction seems to give a correct result:
>lin = ImageAdjust[image, {0, 0, 1.8}];
>res = ImageResize[ImageResize[lin, Scaled[1/2]], Scaled[2]];
>res = ImageAdjust[res, {0, 0, 1/1.8}];
>(These values may be machine and OS dependent.)

Yes, very much so, dependent on the source image, and how you got it into 
Mathematica (copy/paste results from browser are dependent on browser & OS 
used, for example).
My "quick code" assumes strictly SRGB (flat part of the curve accounted for 
instead of pure gamma curve), normalized to [0,1] and, I think, does not handle 
alpha channels properly. :)

ImageAdjust works well if you have just a pure gamma curve (like you would with 
AdobeRGB, for example), or if absolute accuracy isn't too important and 
approximation is good enough (like it is for this test image, actually).

Can you divulge if there would be any plans to implement generalized color 
handling for images in Mathematica?
I.e. something along the lines:
(* assert image colorspace as "xxx")
img=Image[blaa, ColorSpace->"AdobeRGB"];
(* convert to new space *)
ColorConvert[img, "SRGB"]

Preferrably with the possibility of user definable matrices/color 
primaries/transfer functions? (of course, this could be expanded to be usable 
in Import/Export and image functions in general)

I was going to implement something like that (for my own amusement mainly, to 
learn Mathematica and some color theory and to test analyzing some video data 
where it is important to have the BT.601/709 primaries accounted for). This is, 
of course, going to be "slowish" (for "real" use) compared to if internal 
functions would support something like this directly. Compile[] seems to work 
wonders, though (still slow as molasses compared to properly vectorized asm 
implementation, which is not surprising). :)

>Matthias Odisio
>Wolfram Research


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