Particle count on FPGA

Let's say I have a binary image on the FPGA. What's the best way to count how many particles are in the image? Is there a solution that keeps the image on the FPGA?

If the answer is "no", one option is to transfer the image to the CPU. Is the best option for image transfer to use a FIFO? I'm looking to minimize worst case latency. Image size is ~2000 pixels.