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## sdt_dead_reckoning.h - Single header file library to create signed distance fields

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Another small C function that came out of the project I'm currently working on. This one is to create a signed distance field out of a black and white image (see pictures below). Signed distance fields are quite useful for rendering 2D or 3D shapes on the GPU. I came across them while investigating text rendering but they're very versatile constructs.

While searching for a function to create one I stumbled across the paper "The dead reckoning signed distance transform" by George J. Grevera. It contained a quite nice pseudo-code implementation of the algorithm so I ended up implementing it in C. This is how code using it looks like:

``````// Load the mask image with stb_image.h
int width = 0, height = 0;

// Allocate and create the distance field for the mask. Pixels > 16 in the
// mask are considered inside. Negative distances in the distance field are
// inside, positive ones outside.
float* distance_field = malloc(width * height * sizeof(float));

// Create an 8 bit version of the distance field by mapping the distance
// -128..127 to the brightness 0..255
uint8_t* distance_field_8bit = malloc(width * height * sizeof(uint8_t));
for(int n = 0; n < width * height; n++) {
float mapped_distance = distance_field[n] + 128;
float clamped_distance = fmaxf(0, fminf(255, mapped_distance))
distance_field_8bit[n] = clamped_distance;
}
``````

The distance field itself is written into an `float` buffer. I've added the "mapping" part above because in most use cases you want to store the distance field not as floats but as something else (e.g. as 8 bit image). The function leaves that to you because it would make the API quite a lot more bloated.

Below is an example of an 500x500 pixel black and white mask image and the distance field computed for it. On my old Intel Core i5-3570K @3.40GHz it took about 5ms to compute it (single threaded, compiled with `-O2 -ffast-math`). Not useful for fancy realtime stuff but fast enough for my needs. And above all: Simple to use. :)

While it doesn't look like much a distance field is really easy to render with a GLSL shader. And you get high-quality anti-aliasing almost for free. Another very handy thing is that you can shrink or grow the shape by simply adding or subtracting a value from the distance in the shader. And a lot of other cool things.

Without going into to much detail… if you have to render 2D shapes look into signed distance fields. :)

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Anonymous

Hello,

I love your site, man. Its looks like you haven't posted in a while though. I'm really interested in your writings. Please continue to post… Sincerely, Anonymous

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