Pumpkin Carving Patterns from Photos with Julia

Over the last few years, my family and I have rekindled our collective interest in carving jack-o'-lanterns. As a child, I would carve fairly simple templates with my parents, but in 2019 I found a tutorial for creating photo-realistic templates with photo editing software (I used GIMP). The resulting carving of my wife was cool, but it took a lot of manual editing to get to a usable template from a reference photo.

Very spooky!

This year, I decided to see what I could do to automate most of the pattern creation steps using Julia's image processing libraries. Inspired by the grumpy mug of my former foster cat Bruno, I got to work.

This tutorial was made with Literate.jl; you can find the source code here.

My dearly beloved former foster cat Bruno.

We'll only be making use of a few image processing packages.

using Images, ImageFiltering, ImageContrastAdjustment

Here's an outline of the steps I used to create a pattern (we'll look at each in detail shortly):

  1. Manual Background Removal: you can get a fairly good template without removing the background, but I found the contrast was improved by doing this on my iPad first. I leave the implementation of an automatic segmentation algorithm for future work.

Bruno with a black background.

  1. Grayscale: since I can only control how much light gets through the pumpkin, there's no need for colour.

  2. Equalization: this step spreads out the dynamic range of the pixels to create greater contrast for a clearer image.

  3. Smoothing: real photographs contain too much fine detail for a novice carver like me to recreate, so some form of smoothing filter is needed.

  4. Thresholding: once again, I'm not a skilled artist, so I will only be creating a template with three shades (white, black, and gray). It's possible to have a more shades, or even continuous gradients, but that's well beyond my carving abilities.

  5. Manual Touch-up: unfortunately, the process isn't perfect, and some manual changes on my iPad were needed to simplify the template.


Let's load up the image and make it grayscale.

rgb_image = load("_assets/img/blog/pumpkinizer/bruno_pumpkin_black_background.jpg")
gray_image = Gray.(rgb_image);

Grayscale Bruno.


Next, we'll apply histogram equalization. Notice how much this simple step has improved the contrast, which will help us to eventually separate Bruno's feature's into three clear shades.

hist_equal = adjust_histogram(gray_image, Equalization(nbins = 256));

Histogram-equalized Bruno.

Smoothing - Bilateral Filtering

We use a bilateral filter for its edge-preserving properties.

function bilateral_filter(img, n::Int=5, σr=0.5, σd=0.5)
    img_filt = zeros(size(img))
    for i = 1:size(img, 1)
        for j = 1:size(img, 2)
            w_sum = 0.0
            img_sum = 0.0
            for u = max(1, i-n):min(size(img, 1), i+n)
                for v = max(1, j-n):min(size(img, 2), j+n)
                    w_ijuv = exp(-((i-u)^2 + (j-v)^2)/(2*σd^2) - (img[i, j] - img[u, v])^2/(2*σr^2))
                    w_sum += w_ijuv
                    img_sum += w_ijuv*img[u, v]
            img_filt[i, j] = img_sum/w_sum
    return img_filt

The kernel size and smoothing parameters \(\sigma_r\) and \(\sigma_d\) allow for a great deal of control. I eventually settled on the values below.

σr = 3 # Smoothing parameter based on pixel proximity (reduce this for more detail)
σd = 7 # Smoothing parameter based on pixel intensity similarity (reduce this for more detail as well)
kernel_size = 14 # Size of the window to use (make this larger for less detail)
filtered_image = bilateral_filter(hist_equal, kernel_size, σr, σd);

Bruno's face is now blurrier and therefore more easily segmented into simple blobs, but notice how the lines are still fairly crisp (and therefore easy to carve):

Big bad bilaterally-filtered Bruno.

Thresholding Operation

The final step was simply a thresholding procedure so that the pattern only featured three shades. Carving with a greater number of shades is beyond my abilities, but more skilled pumpkin pulp-sculptors can easily modify this procedure to support more detailed shading.

function threshold_image(img, black_threshold, gray_threshold)
    thresholded_image = zeros(size(img))
    for ind in eachindex(img)
        if img[ind] < black_threshold
            thresholded_image[ind] = 0.0
        elseif img[ind] > gray_threshold
            thresholded_image[ind] = 1.0
            thresholded_image[ind] = 0.5
    return thresholded_image

After some experimentation, the following parameters were used:

black_threshold = 0.5 # Increase for more black vs. gray
gray_threshold = 0.75 # Increase this for more gray vs. white
thresholded_image = threshold_image(filtered_image, black_threshold, gray_threshold);

The resulting image is now pretty much ready to be saved (with save("template.png", thresholded_image)) and used as a carving pattern:

Thresholded Bruno.

Manual Touch-Up

The final step before printing and carving involved some manual simplification on my iPad, the re-addition of some whiskers that were lost in translation, and adding some whitespace to save ink.

The final product.

Carving Time

I set up my workspace and taped the template on the nicest face of a newly-gutted pumpkin:

Here we go!

After a few hours of detailed carving, the un-illuminated Brun-o'-lantern looked pretty terrible:

Hours later!

But I was pretty pleased with the final product when lit up, even though I made some mistakes:

My Brun-o'-lantern.

Here's an image of the full pipeline that helps to visualize the changes each step introduces:

Patent patiently pending.

Tips to myself (and others) for improving next year's pumpkin:

  1. I carved the wall WAY too thin! Brun-o'-lantern's mouth fell off and I needed to support it with a pin.

  2. The mouth was also just way too small, I need to make sure shapes have better structural integrity.

  3. I need to make smaller holes in the dotting step - is my hole-poking tool too big, or did I just poke too hard?

  4. Projecting a flat image onto a curved surface is never perfect, but the hole in the middle of his head is mostly my fault.

  5. I need to be choosier with my pumpkin: this one was too small and curved for someone with my skill level.

I hope you found this informative! Feel free to use and modify the source code to make your own pumpkin carving templates next Halloween!