Histogram based on image as vector graphic

Question

I would like to transform histograms based on images to vector graphics.

This could be a start:

function preload() {
  img = loadImage("https://upload.wikimedia.org/wikipedia/commons/thumb/3/36/Cirrus_sky_panorama.jpg/1200px-Cirrus_sky_panorama.jpg");
}

function setup() {
  createCanvas(400, 400);
  background(255);
  img.resize(0, 200);
  var maxRange = 256
  colorMode(HSL, maxRange);
  image(img, 0, 0);
  var histogram = new Array(maxRange);
  for (i = 0; i <= maxRange; i++) {
    histogram[i] = 0
  }

  loadPixels();

  for (var x = 0; x < img.width; x += 5) {
    for (var y = 0; y < img.height; y += 5) {
      var loc = (x + y * img.width) * 4;
      var h = pixels[loc];
      var s = pixels[loc + 1];
      var l = pixels[loc + 2];
      var a = pixels[loc + 3];
      b = int(l);
      histogram[b]++
    }
  }
  image(img, 0, 0);
  stroke(300, 100, 80)
  push()
  translate(10, 0)
  for (x = 0; x <= maxRange; x++) {
    index = histogram[x];

    y1 = int(map(index, 0, max(histogram), height, height - 300));
    y2 = height
    xPos = map(x, 0, maxRange, 0, width - 20)
    line(xPos, y1, xPos, y2);
  }
  pop()
}

<script src="https://cdn.jsdelivr.net/npm/p5@1.4.1/lib/p5.js"></script>

But I would need downloadable vector graphic files as results that are closed shapes without any gaps between. It should look like that for example:

<svg viewBox="0 0 399.84 200"><polygon points="399.84 200 399.84 192.01 361.91 192.01 361.91 182.87 356.24 182.87 356.24 183.81 350.58 183.81 350.58 184.74 344.91 184.74 344.91 188.19 339.87 188.19 339.87 189.89 334.6 189.89 334.6 185.29 328.93 185.29 328.93 171.11 323.26 171.11 323.26 172.55 317.59 172.55 317.59 173.99 311.92 173.99 311.92 179.42 306.88 179.42 306.88 182.03 301.21 182.03 301.21 183.01 295.54 183.01 295.54 179.04 289.87 179.04 289.87 175.67 284.21 175.67 284.21 182.03 278.54 182.03 278.54 176 273.5 176 273.5 172.42 267.83 172.42 267.83 179.42 262.79 179.42 262.79 182.03 257.12 182.03 257.12 183.01 251.45 183.01 251.45 178.63 245.78 178.63 245.78 175.21 240.11 175.21 240.11 182.03 234.86 182.03 234.86 150.42 229.2 150.42 229.2 155.98 223.53 155.98 223.53 158.06 217.86 158.06 217.86 167.44 212.19 167.44 212.19 162.58 206.52 162.58 206.52 155.98 200.85 155.98 200.85 158.06 195.18 158.06 195.18 167.44 189.51 167.44 189.51 177.46 183.84 177.46 183.84 166.93 178.17 166.93 178.17 153.69 172.5 153.69 172.5 155.87 166.82 155.87 166.82 158.05 161.78 158.05 161.78 155.63 156.11 155.63 156.11 160.65 150.84 160.65 150.84 146.59 145.17 146.59 145.17 109.63 139.49 109.63 139.49 113.67 133.82 113.67 133.82 61.48 128.15 61.48 128.15 80.59 123.11 80.59 123.11 93.23 117.44 93.23 117.44 97.97 111.76 97.97 111.76 78.07 106.09 78.07 106.09 61.66 100.42 61.66 100.42 93.23 94.75 93.23 94.75 98.51 89.7 98.51 89.7 85.4 84.03 85.4 84.03 111.03 78.99 111.03 78.99 120.57 73.32 120.57 73.32 124.14 67.65 124.14 67.65 23.48 61.97 23.48 61.97 0 56.3 0 56.3 120.57 50.63 120.57 50.63 167.01 45.38 167.01 45.38 170.83 39.71 170.83 39.71 172.26 34.03 172.26 34.03 178.7 28.36 178.7 28.36 175.36 22.69 175.36 22.69 170.83 17.02 170.83 17.02 172.26 11.34 172.26 11.34 178.7 5.67 178.7 5.67 103.85 0 103.85 0 200 399.84 200"/></svg>

Has anyone an idea how to program that? It doesn't necessarily need to be based on p5.js, but would be cool.

Answer 1

Closing Gaps

In order to have a gapless histogram, you need to meet the following condition:

numberOfBars * barWidth === totalWidth 

Right now you are using the p5 line() function to draw your bars. You have not explicitly set the width of your bars, so it uses the default value of 1px wide.

We know that the numberOfBars in your code is always maxRange which is 256.

Right now the total width of your histogram is width - 20, where width is set to 400 by createCanvas(400, 400). So the totalWidth is 380.

256 * 1 !== 380

If you have 256 pixels of bars in a 380 pixel space then there are going to be gaps!

We need to change the barWidth and/or the totalWidth to balance the equation.

For example, you can change your canvas size to 276 (256 + your 20px margin) and the gaps disappear!

createCanvas(276, 400);

However this is not an appropriate solution because now your image is cropped and your pixel data is wrong. But actually...it was already wrong before!

Sampling Pixels

When you call the global loadPixels() function in p5.js you are loading all of the pixels for the whole canvas. This includes the white areas outside of your image.

for (var x = 0; x < img.width; x += 5) {
    for (var y = 0; y < img.height; y += 5) {
        var loc = (x + y * img.width) * 4;

It is a 1-dimensional array, so your approach of limiting the x and y values here is not giving you the correct position. Your loc variable needs to use the width of the entire canvas rather than the width of just the image, since the pixels array includes the entire canvas.

var loc = (x + y * width) * 4;

Alternatively, you can look at just the pixels of the image by using img.loadPixels() and img.pixels.

img.loadPixels();

for (var x = 0; x < img.width; x += 5) {
  for (var y = 0; y < img.height; y += 5) {
    var loc = (x + y * img.width) * 4;
    var h = img.pixels[loc];
    var s = img.pixels[loc + 1];
    var l = img.pixels[loc + 2];
    var a = img.pixels[loc + 3];
    b = int(l);
    histogram[b]++;
  }
}

The pixel values are always returned in RGBA regardless of the colorMode. So your third channel value is actually the blue, not the lightness. You can make use of the p5.js lightness() function to compute the lightness from the RGBA.

Updated Code

The actual lightness histogram looks dumb because 100% dwarfs all of the other bars.

function preload() {
  img = loadImage("https://upload.wikimedia.org/wikipedia/commons/thumb/3/36/Cirrus_sky_panorama.jpg/1200px-Cirrus_sky_panorama.jpg");
}

function setup() {
  const barCount = 100;
  const imageHeight = 200;

  createCanvas(400, 400);
  background(255);
  colorMode(HSL, barCount - 1);

  img.resize(0, imageHeight);
  imageMode(CENTER);
  image(img, width / 2, imageHeight / 2);
  img.loadPixels();

  const histogram = new Array(barCount).fill(0);

  for (let x = 0; x < img.width; x += 5) {
    for (let y = 0; y < img.height; y += 5) {
      const loc = (x + y * img.width) * 4;
      const r = img.pixels[loc];
      const g = img.pixels[loc + 1];
      const b = img.pixels[loc + 2];
      const a = img.pixels[loc + 3];
      const barIndex = floor(lightness([r, g, b, a]));
      histogram[barIndex]++;
    }
  }

  fill(300, 100, 80);
  strokeWeight(0);

  const maxCount = max(histogram);

  const barWidth = width / barCount;
  const histogramHeight = height - imageHeight;

  for (let i = 0; i < barCount; i++) {
    const count = histogram[i];
    const y1 = round(map(count, 0, maxCount, height, imageHeight));
    const y2 = height;
    const x1 = i * barWidth;
    const x2 = x1 + barWidth;
    rect(x1, y1, barWidth, height - y1);
  }
}

<script src="https://cdn.jsdelivr.net/npm/p5@1.4.1/lib/p5.js"></script>

But the blue channel histogram looks pretty good!

function preload() {
  img = loadImage("https://upload.wikimedia.org/wikipedia/commons/thumb/3/36/Cirrus_sky_panorama.jpg/1200px-Cirrus_sky_panorama.jpg");
}

function setup() {
  const barCount = 100;
  const imageHeight = 200;

  createCanvas(400, 400);
  background(255);

  img.resize(0, imageHeight);
  imageMode(CENTER);
  image(img, width / 2, imageHeight / 2);
  img.loadPixels();

  const histogram = new Array(barCount).fill(0);

  for (let x = 0; x < img.width; x += 5) {
    for (let y = 0; y < img.height; y += 5) {
      const loc = (x + y * img.width) * 4;
      const r = img.pixels[loc];
      const g = img.pixels[loc + 1];
      const b = img.pixels[loc + 2];
      const a = img.pixels[loc + 3];
      const barIndex = floor(barCount * b / 255);
      histogram[barIndex]++;
    }
  }

  fill(100, 100, 300);
  strokeWeight(0);

  const maxCount = max(histogram);

  const barWidth = width / barCount;
  const histogramHeight = height - imageHeight;

  for (let i = 0; i < barCount; i++) {
    const count = histogram[i];
    const y1 = round(map(count, 0, maxCount, height, imageHeight));
    const y2 = height;
    const x1 = i * barWidth;
    const x2 = x1 + barWidth;
    rect(x1, y1, barWidth, height - y1);
  }
}

<script src="https://cdn.jsdelivr.net/npm/p5@1.4.1/lib/p5.js"></script>

Answer 2

Just to add to Linda's excellent answer(+1), you can use p5.svg to render to SVG using p5.js:

let histogram;

function setup() {
  createCanvas(660, 210, SVG);
  background(255);
  noStroke();
  fill("#ed225d");
  // make an array of 256 random values in the (0, 255) range
  histogram = Array.from({length: 256}, () => int(random(255)));
  //console.log(histogram);
  
  // plot the histogram
  barPlot(histogram, 0, 0, width, height);
  // change shape rendering so bars appear connected
  document.querySelector('g').setAttribute('shape-rendering','crispEdges');
  // save the plot
  save("histogram.svg");
}

function barPlot(values, x, y, plotWidth, plotHeight){
  let numValues = values.length;
  // calculate the width of each bar in the plot
  let barWidth  = plotWidth / numValues;
  // calculate min/max value (to map height)
  let minValue  = min(values);
  let maxValue  = max(values);
  // for each value
  for(let i = 0 ; i < numValues; i++){
    // map the value to the plot height
    let barHeight = map(values[i], minValue, maxValue, 0, plotHeight);
    // render each bar, offseting y 
    rect(x + (i * barWidth), 
         y + (plotHeight - barHeight), 
         barWidth, barHeight);
  }
}

<script src="https://unpkg.com/p5@1.3.1/lib/p5.js"></script>
<script src="https://unpkg.com/p5.js-svg@1.0.7"></script>

(In the p5 editor (or when testing locally) a save dialog should pop up. If you use the browser's Developer Tools to inspect the bar chart it should confirm it's an SVG (not <canvas/>))