Re: non-color-scaled rgbimage plots

[Daniel J S. <dan...@ie...>]

Someone on Octave's bug tracker noticed a change in image color scaling 
behavior:

https://savannah.gnu.org/bugs/?52401


On 08/17/2017 03:54 PM, Dima Kogan wrote:
 > Ethan A Merritt <sf...@us...> writes:
[snip]
 >> If backward compatibility with current behaviour is a concern
 >> (not sure it is in this case), then the default could be
 >> to use the cbrange if the user has not set something else.
 >
 > OK, that sounds like a plan. In the meantime I'm going to apply the
 > attached patch to my own builds. This detaches the rgb colors from the
 > palette entirely, both for autoscaling and for rendering. Works OK in
 > initial testing.

This patch/changeset divides by 255.0.  That assumes that the user has 
an image with 8-bit color depth, does it not?  (The cb-autoscaling takes 
assumptions about depth out of the equation.)  What happens if someone 
uses an image format that is different from 8-bit, such as newer JPEG 
with 12-bit depth?

https://www.popphoto.com/news/2014/01/jpeg-standard-91-will-bring-12-bit-color-lossless-compression

https://laurashoe.com/2011/08/09/8-versus-16-bit-what-does-it-really-mean/


On 08/16/2017 01:43 PM, Ethan A Merritt via gnuplot-beta wrote:
> On Wednesday, 16 August, 2017 10:24:16 Dima Kogan wrote:
>> Ethan A Merritt <sf...@us...> writes:
[snip]
> I agree that it makes no sense to apply the palette range (cbrange)
> to rgb data.
> For example this just seems wrong to me:
> 
>      # looks nice
>      plot 'nicepicture.jpeg' binary filetype=auto with rgbimage
>      # messed up
>      set cbrange [50:100]
>      replot
>      # lost altogether
>      set log cb
>      replot

Was it much different than this behavior?

gnuplot> splot sin(sqrt(x**2+y**2))/sqrt(x**2+y**2) with pm3d
gnuplot> splot 500*sin(sqrt(x**2+y**2))/sqrt(x**2+y**2) with pm3d
gnuplot> set cbrange [50:100]
gnuplot> replot

Autoscaling of cbrange is similar to mapping the histogram of the pixel 
colors into the usable range, but often one wants to change that range. 
For example, if one scans a newspaper article it often has yellowish 
tint but expanding the range will map the yellowish background to near 
white but still retain reasonably facsimile of the foreground image. 
XSane image scanner software (see attached) has just such a simple 
scaling to enhance contrast, etc.

https://en.wikipedia.org/wiki/Contrast_(vision)

Sure, one can apply a math formula to the image data input to do 
autoscaling and so on, but the same can be said for pm3d and other modes 
that use cbrange auto-scaling.

Dan