$($Particle Identification$)$ The first challenge is to identify the beads amidst the noisy data. Each image is $640-$by$-480$ pixels, and each pixel is represented by a Color object which needs to be converted to a luminance value ranging from $0\mathrm{.}0($black$)$ to $255\mathrm{.}0($white$).$ Whiter pixels correspond to beads $($foreground$)$ and blacker pixels to water $($background$).$ We break the problem into three pieces: $1.$ Read the image. Use the Picture data type to read in the image. $2.$ Classify the pixels as foreground or background. We use a simple, but effective, technique known as thresholding to separate the pixels into foreground and background components: all pixels with monochrome luminance values strictly below some threshold $($tau$)$ are considered background, and all others are considered foreground. The two pictures in figure above illustrate the original frame $($left$)$ and the same frame after thresholding $($right$),$ using $=180\mathrm{.}0.$ This value of results in an effective cutoff for the supplied data. $3.$ Find the blobs. A polystyrene bead is typically represented by a disc$-$like shape of at least some minimum number pixels $($typically $25)$ of connected foreground pixels. A blob or connected component is a maximal set of connected foreground pixels, regardless of its shape or size. We will refer to any blob containing at least pixels number of pixels as a bead. The center$-$of$-$mass of a blob $($or bead$)$ is the average of the x$-$ and y$-$coordinates of its constituent pixels. Define a data type called BlobFinder in blob$\_$finder.py that supports the following API. Use depth$-$first search to efficiently identify the blobs. Use this data type: Instance variable:$-$ Blobs identified by this blob finder, $\_$blobs $($list of Blob objects$).$ BlobFinder$()-$ Initialize blobs to an empty list.$-$ Create a $2$D list of booleans called marked, having the same dimensions as pic.$-$ Enumerate the pixels of pic, and for each pixel $($i$,$ j$)$:$-$ create a Blob object called blob;$-$ call $\_$findBlob$()$ with the appropriate arguments; and$-$ add blob to blobs if it has a non$-$zero mass. bf$.\_$findBlob$()-$ Base case: return if pixel $($i$,$ j$)$ is out of bounds, or if it is marked, or if its luminance $($use the function luminance.luminance$()$ for this$)$ is less than tau.$-$ Mark the pixel $($i$,$ j$).-$ Add the pixel $($i$,$ j$)$ to the blob blob.$-$ Recursively call $\_$findBlob$()$ on the N$,$ E$,$ W$,$ and S pixels. bf$.$getBeads$($pixels$)-$ Return a list of blobs from blobs that have a mass $$ pixels