Re: [Gimp-user] how can I change the existing values of color and alpha ?

[Ofnuts <ofnuts laposte net>]

On 12/04/2013 12:04 PM, Cristian Secară wrote:
> I have a plain rectangle with overall c09300c0 RGBA levels.
> I want to change the alpha channel value from c0 (193) to some other
> value, for the whole image at once. Or some other color channel.
>
> How can I change the absolute value of a channel ? The color menu has
> only relative adjustments.
>
> Thank you,
> Cristi

To change the alpha value, add a layer mask (initialize by transferring 
the layer's alpha channel), and bucket fill with the required value.

To change a color channel (say, red): use the Levels tool, select the 
red channel, and set it value to 0 everywhere by dragging white handle 
on the bottom scale fully to the right. Then add a layer, set it to 
"Addition" mode, and paint on it using colors with only a red component.