Recall that we have red blood cells, both fixed and living. They average 7.7 microns in size and were photographed unstained with differential interference contrast lighting. The fixed preparation was fairly easy as the cells were isolated (left). The living cells tend to adhere and form rolls (right).

The image is too messy to analyze, even manually. Best we can do is to evaluate the area covered by these cells.

I had to crop the image to ensure reasonable processing times. To try to estimate the area covered by these cells, I did 7 rounds of erosion. Then I analyzed the image with Pixcavator, settings: area = 67,523. The area of the only dark objects (given by the red contour) was 101,250. Considering 7 rounds of erosion, estimate is a bit off. Then the area covered by the cells is 709×619 (the area of the image) - 101,250 = 337,621 pixels.

Based on this data one may try to estimate the number of cells by dividing the found area by the perceived density of the cells. This number would have to be found manually. Considering the fact that the density varies a lot, the resulting estimate would be quite crude.

This is a clear example of limitations of digital image analysis.

For other examples, see our wiki.