Topic awaiting preservation: Thinking about Colour Cast

colour cast = bleh

Mixed blessing: I've been thinking about it all day.

Common problem: white wedding dress has a nasty blue colour cast.

Simple fix:
Sample colour you want to blast.
New layer and fill.
Set blending mode to Colour and bring opacity down to 50%.

Me, being the geek that I am, starting tearing that apart to see the the little pieces.
I started with Colour blending mode, and Colour = Hue & Sat.

Imagine the colour circle.
Pure hues on the outside and the center is sat=0%.
Blue is the colour we want to get rid off.
Since this is inverted, should get some yellow.
On our circle, we have two points: yellow and blue.
The saturation for both points is the same. That is, equidistant from the center.
You can even think of is as another circle within the bigger circle.

The above technique is a way of comprimising between those two points.
If you do a perfect compromise, you end up in the center where sat=0%.

In the above technique, we are compromising Hue & Sat (Colour).
Sat is lowered, but what exactly happens to Hue?
Well, hue doesn't really change much. It's more of a threshold.
As blue gets closer to the center, the hue stays the same and it's only sat that is changing.
When you hit the exact center, hue ceases to exist.
If you keep on going, the hue will suddenly become yellow.

The interesting thing isn't what happens to blue and yellow exactly, but happens to the other hues.

As the blue moves towards the center, hues around blue are squished into blue.
The hues a little farther from blue are expanded to compensate.

The exact hue that you are trying to blast is actually still there in the exact same spot.
It's the hues around blue and the lower sat that give the illusion that it's gone.
Neat.

With this, perhaps a better tool/technique for blasting colour cast be achieved.
How close to blue do you want to squish?
How much further out to exand?
How to deal with saturation?

Oh, I've got some ideas.

LOL, and LOL does not mean that I don't appreciate this, the depth and experimentation level that you strive to achieve with PS amazes me, but you are truly worthy of this place, an asylum.
Your solitary, monologue-ish posts are funny, in the most sincere way.
I mean i can almost picture you walking on long darc coridors or in your cell, from wall to wall, mumbling: Yes, yes, blue can be taken out (shiver), yes, i have an idea... And then you scribble something ont he moist wall with a long finger nail...And you faint after covering the entire wall.



Curiously yours, crip

(Edited by crip on 02-24-2005 18:17)

Not too far from the truth, Crip.
Lately I've been so detached from everything else.
I'll be walking around trying to do some housework, but my mind will wander and I'll forget what I was doing.
Net effect is me just walking around and looking at the floor in a daze.
Yes, the occassional mumble.
And I've filled over 20 pages in my notebook with ideas and other little chunks.
Getting back into the habit of carrying index cards and a pen for the occassion scrawl when my notebook is more than 3 feet away.

This morning I got up and wrote my own motion blur plug-in.
It has linear fall-off and is uni-directional.
As oppossed to Motion Blur, which is a 'flat' kernel and bi-directional.
Minor interpolation problem, though, but I'll eventually get it.

Imagine if you could use a D-Map for the theta/rho of a uni-directional blur.

Damn. I still have to finish my ellipsoid thing.
I have the pieces, but still have to finish cobbling them together.
I really should show ya'll the 3d box thing I did as a preliminary to the ellipsoid.
It's really cool when you combine it with the space converter I did real quck-n-sleazy.

(Edited by warjournal on 02-24-2005 18:58)

Not to mention that I'm starting to think of shifting/offsetting hue in terms of frequency and/or gamma.

:)
Glad to see i was right :)
But, think you could find time to explain the concepts, the idea behind all this, if not actual examples, as in pics?
I mean stuff like: I was wondering if one could do this based on this idea?


Curiously yours, crip

Not sure what exactly you are asking for, Crip.
Right now, this is the best I can offer.

Here is hue squish using sin.
The basic idea is:
h = h + sin(h)*rho

With that trig idea, you get hue squish along these lines:



In that example, the red should be squished. However, the hues around red are squished more than red itself.

I actually like the trig shifting of hue, but that squish needs to be fixed.
Unfortunately, working with trig in FM is tricky.
The in/out ranges are all messed up for FF backwards compatibility and is different for different data types.
Sucks, but I'll get it.

Here are two hue offset 'waves':



The top one is sin().
The bottom one is what happens with the regular colour cast technique.

I'm pretty sure I can find a happy compromise between the two waves.
Prolly be more on the sin() side.

Oh, don't hold me to those waves.
They make sense to me, but might not be entirely accurate in methodology.

Unfortunately, it's not that simple.
Last night I got struck with an idea and it's been burning me up.
I don't know why this didn't occur to me sooner.

Displace moves things around using 2 channels from a PSD.
Consider this: does Displace have to move pixels around?
How about moving colour space values around?

Like this:
channel 1 = hue tweak
channel 2 = sat tweak

If (hue is in this range and sat is in this other range) then (shift hue by this much and shift sat by that much).

In this way, you can do very precise colour manipulation.

A D-Map that manipulates colour space - genius!
Precise intersections with custom fall-offs without the need for ChOps.
Not to mention that you don't have to use hue and sat.
You could very well manipulate two dimensions in any colour space.

All of those D-Map tricks using Curves to manipulate colour channels.
Oh, I'm all goose-pimply.

Got the hue part working real dandy.
There were a few logic bombs that seriously sucked.
But I'm happy with the hue manipulation.

bc_huesquish1.jpg

Just got to add the saturation controls.
Then clean it up.

Got more junk, but gotta go.
Puking kids and all that.

Two plugs that I've been playing with quite a bit lately.

PC/Win
blah blah blah blah

TS_2pack.zip ~164k

RGB 2 Mask

I don't like Select > Colour Range.
I don't like always having to cut-n-paste for some ChOps that I'm fond of.
I don't like this, and I don't like that.

So, I did my own.
It's mini-ChOps with a bit of Channel Mixer tossed in.
Start with an RGB image, convert ranges in the individual RGB channels, then blend them together for a greyscale mask.

Daunting at first, so pay attention.

Let's say you want to select R=50 and G=200.

Red Point = 50
Any R=50 will now be mapped to white.
Red Range will expand that.
Red Range = 10
All R values between 40 and 60 will be mapped to white.
Red Falloff is the... falloff (linear).
Red Falloff = 10
R values between 30 and 40 will be mapped to grey.
R values between 60 and 70 will be mapped to grey.
R values less than 30 will be black.
R values greater than 70 will be black.

Green Point = 50...
and then the Range and Falloff.

Then there is the drop-down.
This is how those things will be put together.
Mixer is kind of like Channel Mixer.
Multiply will multiply them together.
And Screen will screen them together.

Then you have the 3 mixing sliders.
This work for Mixer, Multiply, and Screen.
If you just want R and G to have 'weight', then pump both of them up and lower the B slider.

Tada.

Remind me later to put together a graphic to explain this better.

RGB 2 Mask only works in RGB. I was thinking about adding other colour space channels, but decided on something else.

Enter: Space Converter
This one is easy to figure out.
One of the YUV functions is whack and I haven't figured it out just yet.
Other than that, works dandy.
YUV aside, I'm fairly happy with the 'fidelity' of the conversions.

RGB 2 Mask by itself is pretty cool. It's even cooler if you use Space Converter beforehand. In this way, you can base RGB 2 Mask on HSL and such.

Not quite done with RGB 2 Mask, though. I still have to add the ellipsoid junk, and add a few other things. Not sure when that's going to happen, so there you have it.

Too tired to digest this for the moment.
will look at it another day.

This might be known thinks for you, but I thought "stroker!" when I saw them, so there it is:

http://www.graphics.com/modules.php?name=Sections&op=viewarticle&artid=151

http://www.graphics.com/modules.php?name=Sections&op=viewarticle&artid=153

and

http://www.graphics.com/modules.php?name=Sections&op=viewarticle&artid=153

Pierre Courtejoie

Oh, thanks, Spec. Now I can't get away from this sickly thread.

TS_huesquish.zip ~117k

That's the Hue Squish that I've been playing with. There is a PSD in there with a colour wheel for playing with. It will help get to know how the sliders work.

Hue: The hue to be squished. The other side will be expanded.
Steep: How much expand/contract is to be applied.
Range: The mid-point between expand/contract.
Offset: Offset's the final hue.
Just Hue: A checkbox for... just hue.

Tried it out on some bad hue photos and it seems to be working just as intended. I am very pleased.

Okay, XYZ.
Very fascinating stuff. I just wish it was easier to visualize it in 3d space.
One cool thing is that it is the link between a lot of things. It's the hub or something.
I read somewhere that X+Y+Z=1 or something like that.
Or maybe it was some other flavor of XYZ.
If that property is special in some way, I might have some new junk to play with.
I've just started doing some poking around and I haven't noticed anything just yet.

* For those that don't know, I also go by Stroker. Sometimes even Stroker Lightyear of Borg.