Unless the user is extremely meticulous in the selection of all background colors, the image produced by background subtraction will still have pixels in the background with non-zero alpha.
This will cause blob detection to pick up blobs in the background, but can be mitigated by the addition of a step between background subtraction and blob detection which cleans up the image by increasing the contrast of the alpha channel.
Increasing the contrast of the alpha layer by 5% will probably be sufficient.
Unless the user is extremely meticulous in the selection of all background colors, the image produced by background subtraction will still have pixels in the background with non-zero alpha.
This will cause blob detection to pick up blobs in the background, but can be mitigated by the addition of a step between background subtraction and blob detection which cleans up the image by increasing the contrast of the alpha channel.
Increasing the contrast of the alpha layer by 5% will probably be sufficient.