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Image processing to add alpha channel
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Input: Two photos of the same semi-transparent object on different solid-colored backgrounds

Input images

Output: A single image with the background removed and the correct opacity for each pixel

Output image

The algorithm is as follows:

Detect edges

  1. Call getEdgeFilter to generate a 1D edge detection kernel. It's a gaussian kernel that switches from positive to negative at the center.
  2. For each image, combine the absolute values of the horizontal and vertical convolutions with this kernel (using grayscale versions of the images).
  3. Add a vignette effect using getSpotlight so that edges near the center have more weight.

Here is the result of edge detection, with the color curve adjusted for visibility:

Edge-filtered images

Align images (translation only)

  1. Use the 2D convolution of the two edge-filtered images to find the best alignment.
  2. Save the 2D convolution as alignmentScores.png to allow human inspection.
  3. Crop the images to the area where they intersect when aligned.

Alignment scores

Calculate axis of color difference

Our code still doesn't know the background color of each image. First we will find the direction of the color-space vector connecting the two background colors. This vector is called the "hub", and its direction is called the "hub axis". In theory, the line connecting each pair of corresponding pixels should be parallel to the hub axis.

Poles diagram

Intuitively, we just want to compute the "average direction" of the lines between corresponding pixels. getHubAxis does this in a way that's robust to lighting differences between the two images:

  1. Compute the difference of the two images and center it at the origin of color space.
  2. For each of the resulting color vectors x, compute the outer product xTx
  3. Sum all the outer products into a single 3x3 matrix.
  4. The hub axis will be the eigenvector of that matrix with the largest eigenvalue.

Calculate min and max color difference

In theory, opaque pixels should be the same color in both images. Unfortunately, changing the background color tends to change the lighting of the photo, resulting in this situation:

Opaque pixel has different colors in each photo

Looking at a histogram of color differences along the hub axis, we can see two peaks representing full transparency and full opacity:

Histogram of color differences

Note: The color differences are negative because of an arbitrary choice when calculating eigenvectors. Regardless, values further from the origin are more transparent.

getTransparentDiff and getOpaqueDiff attempt to locate these peaks. The algorithm is neither theoretically sound nor very robust:

  1. Imagine truncating the histogram at each difference value and replacing the rest with a mirror image
  2. Choose the optimal value by minimizing variance divided by area

Using the algorithm to find the full transparency peak

Here is the result:

Cost curves for both peaks

Note: In all of these calculations, the pixels are weighted using a vignette so the center of the image counts more.

Calculate hub center

Now that we know the direction and length of the hub, we just need to find its position in color space.

  1. Call getMountainWeights to weight each pixel by how close it is to full transparency, using an exponential dropoff
  2. Pass those weights into getHubCenter to find the average color of fully transparent pixels across both images. This gives us the center of the hub.

Calculate alpha

Opacity is just a linear function of color difference along the hub axis. The alpha channel is saved as a grayscale image called alpha.png.

Alpha channel of output

Calculate true colors

The basic idea for finding the true colors of the original image is explained in the first diagram on this page. However, instead of using the two input colors directly, we calculate it using the average of the two images along with with the alpha channel from the previous step.

There's a bit of extra math to keep the result from going outside the allowed color range so we don't have to truncate each color channel separately, which might lead to washed out areas.

The output image is saved as transparentImage.png. Here it is with the transparency removed, showing the true colors:

Output without transparency

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