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Blurs an image and downsamples it.

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cv2.pyrDown(src[, dst[, dstsize[, borderType]]]) → dst
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The function performs the downsampling step of the Gaussian pyramid construction. An image pyramid is a collection of images - all arising from a single original image - that are successively downsampled until some desired stopping point is reached. First, it convolves the source image with the kernel: $$\frac{1}{256} \left[ \begin{array}{ccccc} 1 & 4 & 6 & 4 & 1 \\ 4 & 16 & 24 & 16 & 4 \\ 6 & 24 & 36 & 24 & 6 \\ 4 & 16 & 24 & 16 & 4 \\ 1 & 4 & 6 & 4 & 1 \end{array} \right]$$ Then, it downsamples the image by rejecting even rows and columns.

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