[Gimp-developer] what algorithm is used in Ctrl-K?

[saulgoode at flashingtwelve.brickfilms.com]

Quoting David Gowers <00ai99 at gmail.com>:

> On 3/3/07, saulgoode at flashingtwelve.brickfilms.com <
> saulgoode at flashingtwelve.brickfilms.com> wrote:
>>
>> Quoting zhang chi <zhangchipr at gmail.com>:
>>
>>>
>> http://www.gimptalk.com/forum/topic/My-First-Tutorial-Blending-Images-In-Gimp-106-1.html
>> <gimp-developer at lists.XCF.Berkeley.EDU>
>>>
>>> in this tut, after feather,C-i,and C-K is used, but what algorithm is
>> used
>>> when using Ctrl-k ?
>>>
>>> thank you <gimp-developer at lists.XCF.Berkeley.EDU>
>>
>>
>> If the layer has an alpha channel then the value of the alpha is
>> scaled by (1 - S) where "S" is the selection mask's value (which spans
>> the range '0=not selected' to '1=fully selected' -- the selection is
>> actually a value from 0 to 255).
>>
>> If there is no alpha channel then the value of each of the components
>> (RGB) is scaled by (1 - S) and added to the product of the
>> corresponding component of the active background color and (1 - S).
>>
>> For Indexed Mode images, the resulting pixel is the active BG color if
>> S>0.5 and unchanged if S<0.5
>
>
> For Indexed Mode Images, the resulting pixel is the active BG color if the
> layer has no alpha and S > 0.5, (emptiness) if S > 0.5 and the layer has an
> alpha channel, and unchanged if S < 0.5.

Oops, forgot that case.

Also, there is an error in my description:

>> If there is no alpha channel then the value of each of the components
>> (RGB) is scaled by (1 - S) and added to the product of the
>> corresponding component of the active background color and (1 - S).

The very last term in the sentence should be "S" -- not "(1 - S)".