How to adjust RGB saturation?

[rfmmars]

Take a look at this one.

http://neuron2.net/ipw-web/bulletin/...opic.php?t=593

richard
photorecall.net

[Wilbert]

One thing I noticed with Graft's hue filter (Fdump's filter is based on it, since it used the same transformations) is the following:

Start with pure yellow (avs->blankclip). If you open the script in VDub, use Graft's hue filter and set the saturation to zero, you should get pure white. But you get R=G=B=234 instead. Perhaps it's because of the approximate transformations, perhaps there's an error in it. I really want to know how those transformations are derived.

[fccHandler]

Thank you for your work! It will be great having DG's Hue (with RGB support) ported to Avisynth.

BTW, I don't think removing the saturation from yellow makes it white. I'm thinking of a black and white TV show for instance. All the colors reduce to various shades of gray when the color is removed, but I would expect only pure white to actually stay white.

[Wilbert]

BTW, I don't think removing the saturation from yellow makes it white. I'm thinking of a black and white TV show for instance. All the colors reduce to various shades of gray when the color is removed,

Have a look at the diagram in

http://www2.ncsu.edu/scivis/lessons/...els2.html#hue.

Pure yellow (or green, blue, red, etc ...) has an value (lightness) of 1. This implies that reducing the saturation turns it into white.

Now, if the value is less than 1 (r=200,g=200,b=0 for example), then reducing the saturation turns it into grey.

some formulas:

http://en.wikipedia.org/wiki/HSV_color_space
http://www.easyrgb.com/math.php?MATH=M21#text21

[fccHandler]

Originally posted by Wilbert
Pure yellow (or green, blue, red, etc ...) has an value (lightness) of 1. This implies that reducing the saturation turns it into white.

Ah, I see. Based solely on the diagram I must agree with you. But those diagrams and links are all about computer graphics. Maybe it works differently in the video world?

Honestly, "Hue" and "Value" (or "Intensity") I couldn't care less about. I only want the "Saturation" control. In my mind (not getting too technical), I've always thought about it like the "Color" knob on a TV set. You turn it up and colors bloom and people's faces turn red. You turn it down and it's just like watching a B&W set. The "saturation" slider in Donald Graft's filter seems to accomplish a similar effect for digital video, but is "saturation" really the proper term for it?

Just to confuse you more, here's something I noticed: Try desaturating the same BlankClip in VirtualDub, first using Donald's "H/S/I" filter, and then using VirtualDub's internal "HSV adjust" filter. The results are quite different.

Since DG is currently unavailable, perhaps you can ask Avery Lee if he has any thoughts on this.


EDIT:
Just thought of something else. IIRC in video there is the concept of colors being "NTSC safe." (There may also be a similar concept in PAL video.) A vectorscope is a device used to measure the color saturation of broadcast video, and determine if it is "safe" or not. The colors must fall within certain fixed boundaries. Therefore it may not be possible to represent "pure yellow" (or any other pure color) in the video world, i.e., it's always less than Value=1. Perhaps the VirtualDub filters are using formulas customized to reflect this concept...

[Leak]

Originally posted by fccHandler
Since DG is currently unavailable,

This may be a bit offtopic, but is anybody else finding Donald's website to be AWOL?

Not Found
The requested URL / was not found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

Apache/1.3.31 Server at neuron2.net Port 80

np: Autechre - Maphive 6.1 (EP7)

[BruceL]

@Leak
I am getting the same as you at all my neuron2 links. Sure hope he is OK and his site is just experiencing a short term technical problem.

[State of Mind]

Fcchandler, would it be too much trouble for you to post some screenshots of unfiltered DV and enhanced DV in your case? I am interested in seeing the difference in color levels.

[Wilbert]

I'm so confused

Regarding Graft's hue filter. The saturation setting does the following. Setting it to one leaves the rgb clip untouched, and setting it to zero sets the rgb clip to (r,g,b)=(y,y,y) where y is the luminance (as in yuv). In [0,1] it just performs a linear interpolation (and linear extrapolation outside [0,1]).

Normally we have y = 0.299*r + 0.587*g + 0.114*b, but they use a gamma-corrected version: y = 0.3086*r + 0.6094*g + 0.0820*b (don't know yet where these numbers are coming from ...).

http://www.sgi.com/misc/grafica/matrix/index.html

What I don't understand is, how is this related to the saturation in the hsv color format? I guess there's no relation? Setting the saturation to zero results in (r,g,b) = (V,V,V) with V = max(r,g,b) and setting it to one the (r,g,b) may or may not change.

[Wilbert]

@fccHandler,

First test version:

http://www.wilbertdijkhof.com/Hue_v10.zip

It should produce identical results.

I also want to look at the hue implementation of the internal vdub hsi filter. Adjusting hue is implemented correctly here.

[fccHandler]

Originally posted by State of Mind
Fcchandler, would it be too much trouble for you to post some screenshots of unfiltered DV and enhanced DV in your case? I am interested in seeing the difference in color levels.

If you have VirtualDub and Donald Graft's Hue/Saturation/Intensity filter, you can make your own screenshots. The only parameter I'm changing is the saturation. Even at 83% the difference is visually subtle, but it can be objectively measured with trevlac's excellent "Color Tools" filter (in vectorscope mode).

[fccHandler]

Originally posted by Wilbert
First test version:

http://www.geocities.com/wilbertdijkhof/Hue_v10.zip

It should produce identical results.

Great work! I'm sure I'll be testing this later today.

Just to be complete, I noticed that the original H/S/I filter GUI has a checkbox "Preserve luma on hue change," but I don't see that option in the docs of your version.

I also want to look at the hue implementation of the internal vdub hsi filter.

I looked at it. But typical of Avery, it's so heavily optimized as to be incomprehensible to mere mortals.

[Wilbert]

Just to be complete, I noticed that the original H/S/I filter GUI has a checkbox "Preserve luma on hue change," but I don't see that option in the docs of your version.

I still have to add this option

[Overdrive80]

I resurrected the thread to request that the last option of the filter be finished. Thanks.