Re: non-color-scaled rgbimage plots

[Ethan A M. <sf...@us...>]

On Tuesday, November 14, 2017 2:12:05 PM PST Daniel J Sebald wrote:
> On 11/14/2017 03:19 PM, Ethan A Merritt wrote:
> > On Tuesday, November 14, 2017 12:03:09 PM PST Daniel J Sebald wrote:
> >> On 11/14/2017 12:27 PM, Ethan A Merritt wrote:
> >>> There are no "rgb axes".
> >>> And no, it does not make sense to autoscale RGB components of an image.
> >>> Suppose you are displaying a photograph that for whatever reason does
> >>> not contain any regions with Green==0.   Rescaling the Green component
> >>> would distort all the colors everywhere, leaching the green out of things
> >>> that really are green.  The _representation_ requires the range to run
> >>> [0:255] even if this particular image doesn't happen to contain pixels
> >>> with small Green component values.
> >>
> >> rgb components aren't necessarily treated independently.  This code
> >>
> >> -  image[i_sub_image++] = cb2gray( points[i_image].CRD_R );
> >> -  image[i_sub_image++] = cb2gray( points[i_image].CRD_G );
> >> -  image[i_sub_image++] = cb2gray( points[i_image].CRD_B );
> >>
> >> was combining all component values into one.
> > 
> > You lost me.
> > That code, which no longer exists, was copying the R G and B components
> > sequentially.  They were all being scaled by the same range from the
> > palette definition, which was weird since they are not palette colors.
> > There was no "combine into one".
> 
> Right, I'm pointing that out.  All were scaled with the same formula. 
> You initially gave a counter example saying that if the Green channel is 
> an all zero channel it is going to leach across color channels in some 
> way.  There's nothing weird about the use of cb2range.  The palette 
> didn't apply, not until someone wanted to combine palettes and images. 
> The cb2gray() is just a linear transformation using the parameters 
> cbaxis->min and cbaxis->max.  Call the function something more 
> generalized, linearmap(), whatever.  And if the code were more 
> object-oriented, the linearmap() routine could be applied to xyz-axis, 
> colorbar, alpha channel, rgb, i.e., code reuse.
> 
> In terms of syntatx, it's more confusing to not continue the paradigm 
> from one axis type to another.

We'll have to agree to disagree on that point.
I find it strange to consider RGB color assignments an "axis" at all.

Anyhow, I believe that the most common transformations applied to
color components are nonlinear and cannot be applied to a single
component in isolation, e.g. gamma-correction, color balance,
visual temperature.

min/max linear scaling is not a good model for color manipulation.

    Ethan