Help request: Editing Masks

[Dik Takken]

Hi,

I'm currently trying to do smart unsharp masking in Krita 1.6.3. This 
means that you apply unsharp masking only to those parts of the image that 
contains edges, thereby preventing the unsharp mask from amplifying noise 
in otherwise smooth color gradients.

I tried the following steps:

* Load an image
* Duplicate layer
* Do edge detection
* Use the Levels tool to make all edges bright white
* Blur the result somewhat
* Select highlights
* Create mask from selection

The result is that all detailed areas are masked. They look white, while 
the original image is visible from the untouched bottom layer.

Now I want to manually fine-tune the mask to cover all detailed areas I 
want to perform unsharp masking on. So, I toggle the 'Edit Mask' flag and 
grab the freehand drawing tool.

When I start paiting on the mask, I don't understand the results:

* When painting black, I can erase the white masked areas, revealing the 
image from the bottom mask. Seems reasonable.

* When paiting white, I get black areas on the image, just like I am 
paiting on the layer in stead of the mask. I don't understand this one. I 
was expecting to add to the white areas of the existing mask....

So, erasing parts of the mask works, but how am I supposed to extend the 
mask to cover more area?

Thanks for any hints,

Dik