On Affinity Photo I want to export an entire black image to a 3x3 square of slices. As this square is bigger than original image, 8 exported slices but the center's one have extra pixels. When I exported slices, these extra pixels are white. I want them to be black. I tried many tricks without success: change background color of image, add a square below the image and fill it with black... No success. Extra pixels are always white. Is there a way to achieve this goal?
Original image
Mosaic of slices
Slices
As you can see, extra background is white.
Top left slice