How do I change colors below a level to transparency

Question

I have an image with a black shape in the middle. The surrounding area is white with varying shades of gray speckles surrounding it. I used the Gimp eraser to remove most of the surrounding image. I did a color to alpha filter to the rest and it worked to 90%. When I export the work to a PNG I notice there is gradient noise / artifact in the png in the shape of what I left around the black shape with the erasure. It is a thin film of gray shading. How do I remove that last artifact of the shades of gray splotches? Thank you.

Answer 1

• Layer ➤ Mask ➤ Add layer mask, and initialize to Transfer layer's alpha channel
• Start the Levels tool (which will act on the mask), and drag the leftmost handle in the "Input" area your grey shade disappears.
• You can use the middle handle of the same area to adjust the pixellation of the edges.
• When happy with the result use Layer ➤ Mask ➤ Apply layer mask.

But this problem stems from something you missed earlier. You should probably have removed another color in the color-to-alpha step. Or if there is a pattern in the background, you can try to replicate the complete pattern on an additional layer that you then put in "Color Erase" mode.