A color wheel oddity

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davidh
Posts: 508
Joined: June 9th, 2009, 2:16 am

A color wheel oddity

Post by davidh » October 20th, 2018, 7:59 am

It seems that the color wheel is sometimes not sensitive enough to varying positions of a control point. Or the channel values are not always updated accordingly.

1. open Tint and change one of the colors
2. save the color line and click Tint OK
3. reopen the Tint
4. display the same color picker as in the step 1
5. move the color point on the color wheel a bit so as the color changes but not any of the values of the three channels, see the middle image in the attachment. (It seems to depend on the location of the color point on the color wheel).
6. click OK OK to confirm and close the Tint dialog
7. reopen the Tint dialog and color picker again -> the color point on the color wheel has reverted to its saved position (steps 1-2), see the image on the right in the attachment.

Even if you save the changed color line and confirm the settings with Tint OK, next time you open the Tint dialog the color point will be back at its initial position.

You will not be able to reproduce this if any of the channel values changes while you move the color point along the color wheel in the step 6.


Sometimes the difference between the colors is quite important, so when making small changes, it is necessary to keep an eye on the channel values to make sure that at least one of them changes.
Attachments
Color Picker reopen.png
Color Picker reopen.png (82.92 KiB) Viewed 110 times

jsachs
Posts: 1463
Joined: January 22nd, 2009, 11:03 pm

Re: A color wheel oddity

Post by jsachs » October 20th, 2018, 8:45 am

This is an artifact of the conversion from HSV to RGB and back. Since you have the brightness slider at almost 0, the HSV color hexcone compresses all the colors together so it becomes possible to change H and S without affecting the resulting RGB values. In the extreme case, if you set V to zero, it does not matter at all what H and S you select. If you increase the V slider, you will find any change in the position of the HS point will affect the RGB output. There can also be some very small roundoff error from the conversion from RGB to HSV and back to RGB. None of these errors should make a significant visual difference.

I did make a minor change imposing a minimum size for the color picker so you should always be able to select RGB values without gaps.
Jonathan Sachs
Digital Light & Color

davidh
Posts: 508
Joined: June 9th, 2009, 2:16 am

Re: A color wheel oddity

Post by davidh » October 21st, 2018, 10:08 am

... If you increase the V slider, you will find any change in the position of the HS point will affect the RGB output...
This explains why I never encountered this effect with the brightness slider set to higher values.

...None of these errors should make a significant visual difference....
Not significant but, when used for example with Tint, the difference might be noticeable in lighter areas of an image.

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