Nirav Patel wrote:
> For vision purposes, it would be very useful to have thresholding with
> both upper and lower boundaries, returning both the number of pixels
> within the threshold and the centroid of those pixels. This is a
> trivial addition to the existing transform.threshold() function, but
> is it acceptable to modify the input options and the output of an
> existing function? Would it break compatibility with existing pygame
> games? Would it make sense to have a second function so similar to an
> existing one?
transform.threshold... does not work the way you think it does. It's
not a simple computer imaging "below or above value X" threshold, it's
"are we within X or color Y". You can approximate a two-level threshold
with something like this:
def simple_threshold(surface, min=0, max=255):
dest_surface = pygame.Surface(surface.get_size(), 0, surface)
avg = (min + max) // 2
dist = avg - min
color = (avg, avg, avg)
threshold = (dist, dist, dist)
pygame.transform.threshold(dest_surface, surface, color, threshold)
return dest_surface
Really, the best thing to do might be to create a
transform.grayscale_threshold function. As you can see, the current
threshold function doesn't exactly do what you need, even though the
name matches.
As to the centroid, that might be best as its own function.
> The other function, which is also similar (and could even just be an
> option in thresholding), is thresholding with connected component
> detection. This would involve supplying an upper and lower threshold,
> a Surface, and optionally a mask. The function would find the largest
> blob of pixels in the Surface within the threshold, make a mask of
> those pixels if desired, and return the centroid and number of pixels
> in the blob.
Again, separate out your pieces. What you're describing is
bw_threshold, followed by connected_components and get_centroids.
> It could also be useful to have multiple connected component
> detection, for "multi-touch" without having to use different colored
> objects (or if you are using IR LEDs like the Wii does), but I'm not
> sure how to handle that in a single pass of the array. Actually, I'm
> not really sure how I'm going to handle both detection and creating a
> mask in a single pass either. It may be necessary to store the
> starting pixel, ending pixel, and size of each connected component on
> the first pass, keeping track of which was the largest yet, and then
> have a shorter second pass to create the mask that only starts at the
> starting pixel and ends at the ending pixel.
Go look up the standard connected components algorithm. It's two-pass,
by necessity. If you try to do it in a single pass, you end up with an
O(n^2) algorithm instead of O(n).
What connected components does is take a BW image and turn it into a
grayscale image, with each color of pixel representing a different
connected component. So starting color is irrelevant, since you have to
process it down to BW to start with.
-FM
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