While I'm writing some new content for Core Image for Swift discussing histograms, I've found Core Image's own histogram display filters slightly wanting. Always eager for a challenge, I thought I'd write my own component for displaying histograms and plug it into Filterpedia. The component has no dependencies on Filterpedia, so can easily be reused in any other projects. The component is named HistogramDisplay and uses vImage to calculate the histogram data for a given CGImage. The histogram data consists of four arrays (one array for each color channel) of 256 unsigned integers - each containing the number of pixels in the image with that particular value. With that data, I use my Hermite smoothed drawing code to create Bezier paths for the three color channels (ignoring alpha) and simply use those paths to draw onto CAShapeLayers. The vertical scale is based on the maximum pixel count for any color / tone "bucket". Since there are occasionally outliers that make the scaling a bit crazy, my component allows the user to set the vertical scale with a touch gesture. To open the histogram display as a popover in Filterpedia, toggle the UISwitch in the top right corner of the user interface:
Maybe not the most elegant user interface solution, but working for my immediate requirements. The source code for HistogramDisplay is available here.
It's been a fairly busy few months at my "proper" job, so my recreational Houdini tinkering has taken a bit of a back seat. However, when I saw my Swarm Chemistry hero, Hiroki Sayama tweeting a link to How a life-like system emerges from a simple particle motion law, I thought I'd dust off Houdini to see if I could implement this model in VEX. The paper discusses a simple particle system, named Primordial Particle Systems (PPS), that leads to life-like structures through morphogenesis. Each particle in the system is defined by its position and heading and, with each step in the simulation, alters its heading based on the PPS rule and moves forward at a defined speed. The heading is updated based on the number of neighbors to the particle's left and right. The project set up is super simple:
Inside a geometry node, I create a grid, and randomly scatter 19,000 points across it. An attribute wrangle node assigns a random value to @angle:
@angle = $PI * 2 * rand(@ptnum);
The real magic happens inside another attribute wrangle inside the solver. In a nutshell, my VEX code iterates over each point's neighbors and sums the neighbor count to its left and right. To figure out the chirality, I use some simple trigonometry to rotate the vector defined by the current particle and the neighbor by the current particle's angle, then calculate the angle of the rotated vector.
Not quite finally, because to make things pretty, I update the color using the number of neighbors to control hue:
@Cd = hsvtorgb(N / maxParticles, 1.0, 1.0);
Easy! Solitons Emerging from Tweaked Model
I couldn't help tinkering with the published PPS math by making the speed a function of the number of local neighbors:
@speed = 1.5 * (N / maxParticles);
In the video above, alpha is 182° and beta is -13°. References Schmickl, T. et al. How a life-like system emerges from a simple particle motion law. Sci. Rep.6, 37969; doi: 10.1038/srep37969 (2016).
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