An image for demonstration is shown in screen scr11 as below. It seems that it's background is black. However, it is not exactly what you see.
By touching a "black" pixel, it's RGB color is shown as in screen scr12, scr13.
With this tool, we are going to turn the background to completely black. We use yellow background first so that we can see the changes easier.
We define black color in "Replace color" in screen scr14 and set the "bg color" as yellow in screen scr15. Notice that "Replace color variation" is set to 15 in scr14. If exact "black" color is replaced as "yellow", not many pixels are included. Therefore, a self-defined value 15 is used so that some pixels "seem" to be black are also changed.
Then "Replace" is select to proces.
Screen scr16 shows the image after processing. If users are satisfied, they can change the background back to black color and save the image. If not, the image can be further fine tuned.
When we zoom the image (in view mode), we find that there are still some pixels in screen scr17 should also be replaced.
We touch the pixels that we want to replace and repeat the "replace" process serval times to clean up the edges. We can also change to "edit" mode and adjust the pen size to clean up by touching the pixels.
When We are satisfied, we can change the yellow background to black and the final image is created.