Can I Subtract a color overall?

All -

I don't know if this makes conceptual sense, but ...

I am printing a scanned image onto a slightly toned surface (heavy
watercolor paper), and I would like to precompensate somewhat for the
effect of printing onto this warm toned medium. How can I correct the
image before printing to approach the same colors as a print on white
paper, at least in the darker areas?

Thanks for any comments.

- John Mills

On Sun, Jan 31, 2010 at 12:50 PM, John Mills <johnmills@speakeasy.net> wrote:
> All -
>
> I don't know if this makes conceptual sense, but ...
>
> I am printing a scanned image onto a slightly toned surface (heavy
> watercolor paper), and I would like to precompensate somewhat for the
> effect of printing onto this warm toned medium. How can I correct the
> image before printing to approach the same colors as a print on white
> paper, at least in the darker areas?

I suggest trying this:

1. Create an image full of the color of the paper.
2. Look at the RGB values of the color you chose (my test color was
244, 242, 219)
3. Open up the 'Levels' tool. select Red channel, type the appropriate
value (eg 244) in the rightmost field under the 'input levels'
histogram+gradient. select green, type appropriate value (eg 242),
select blue, type appropriate value (219), OK.
4. The canvas should now be completely white #FFFFFF / 255, 255, 255,
and you should have an appropriate Levels preset stored to apply to
your pictures; you may want to save the preset permanently with the
'+' button next to the preset selector (after first selecting it)


Note:
This method will clip out detail of colors that are as bright or
brighter than the real paper color. This is basically unavoidable
according to your description of the problem.

HTH,
David

Thanks to Stefan, Dan, and David for your replies. I'll look into all
three approaches, but this looks most directly like what I had in mind.

My limited intuition seemed to suggest that since RGB is an additive
color space, there might be a direct way to subtract a cast out of it.

Regards.
- John Mills

On Sun, 31 Jan 2010, David Gowers wrote:

> On Sun, Jan 31, 2010 at 12:50 PM, John Mills <johnmills@speakeasy.net> wrote:
...
>> I am printing a scanned image onto a slightly toned surface (heavy
>> watercolor paper), and I would like to precompensate somewhat for the
>> effect of printing onto this warm toned medium. How can I correct the
>> image before printing to approach the same colors as a print on white
>> paper, at least in the darker areas?
...
> I suggest trying this:

> 1. Create an image full of the color of the paper.
> 2. Look at the RGB values of the color you chose (my test color was
> 244, 242, 219)
> 3. Open up the 'Levels' tool. select Red channel, type the appropriate
> value (eg 244) in the rightmost field under the 'input levels'
> histogram+gradient. select green, type appropriate value (eg 242),
> select blue, type appropriate value (219), OK.
> 4. The canvas should now be completely white #FFFFFF / 255, 255, 255,
> and you should have an appropriate Levels preset stored to apply to
> your pictures; you may want to save the preset permanently with the
> '+' button next to the preset selector (after first selecting it)

> Note:
> This method will clip out detail of colors that are as bright or
> brighter than the real paper color. This is basically unavoidable
> according to your description of the problem.

> HTH,
> David
>

On Mon, Feb 1, 2010 at 1:05 AM, John Mills <johnmills@speakeasy.net> wrote:
> Thanks to Stefan, Dan, and David for your replies. I'll look into all three
> approaches, but this looks most directly like what I had in mind.
>
> My limited intuition seemed to suggest that since RGB is an additive color
> space, there might be a direct way to subtract a cast out of it.
If what I provided is not satisfactory,
you might want to try the inverse transformation (in which the values
go in the 'output maximum' field instead of the 'input maximum'
field.).I do think that the inverse transformation is better for
previewing how a given picture will look on the colored paper than
actually preparing it for print.