I do not know what you mean by 'modify an image dynamically'. Scale an image with an ellipse line, does not look good (the line changes in size). Draw a line by canvas... is what I want, but I do not know how to make an ellipse.
For now I have this: drive.google.com/open
Draw hundreds of 3x3 pixel images to make the line.
To calculate the radius from the planet X/Y, I do it by brute force...