Plotting an outline

I trying to search a suitable number of y scan lines and find the x coordinates of the .bmp picture of the bone of the skull then plot them in OpenGL the outline of the skull. I also want to provide in the software the facility to increase the scan resolution and thus the accuracy of the plot.

I think it easier to edit the bitmap with some kind of paint program and use alpha values to mark the bone. OpenGL can maginfy and filter the picture but the “real” accuracy is depending on the original picture.