If you want to edit the Linear Gradient Mask with the brush by either adding or removing areas of the mask, it will need to be rasterized first. This operation basically changes the mask to be pixel-based, meaning that you cannot readjust the gradient after the fact. The resulting mask will then be the same type as if you were using the Draw Mask feature (or created a Gradient Mask in Capture One 11 and earlier).
- Go to the Layers tool and select the layer with a Linear Gradient Mask.
- Long-press the Draw Mask icon and select Draw Mask (B) or Erase Mask (E) from the drop-down menu or use the keyboard shortcuts B or E.
- Click on the image in the viewer to start editing the mask. Capture One will now alert you that you are about to rasterize the mask. Accept by clicking on Rasterize.
- You can now add or remove areas by painting with the brush, depending on whether you have chosen Draw Mask (B) or Erase Mask (E).
Note that you can also rasterize the Linear Gradient Mask directly by right-clicking on the layer in the Layers tool and selecting Rasterize Mask or choosing Layer -> Rasterize Mask.