An histogram equalization is a way to enhance local contrasts in an image by using an histogram of its values. I explain the idea in this blog post Here I use d3.scale.quantile for bucketing, inspired by these nice algorithms explained here.
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March 12, 2019 09:33
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Histogram Equalization
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| <!DOCTYPE html> | |
| <head> | |
| <meta charset="utf-8"> | |
| <script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.5/d3.min.js"></script> | |
| <style> | |
| body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; } | |
| canvas { width:50%; float: left} | |
| </style> | |
| </head> | |
| <body> | |
| <canvas id="imageOriginalCanvas"></canvas> | |
| <canvas id="imageCanvas"></canvas> | |
| <script> | |
| var cnv = document.getElementById('imageCanvas'); | |
| var ctx = cnv.getContext('2d'); | |
| var cnvori = document.getElementById('imageOriginalCanvas'); | |
| var ctxori = cnvori.getContext('2d'); | |
| var colorScales = { | |
| 'linearBlackAndWhite': function(values){ | |
| return d3.scale.linear() | |
| .domain(d3.extent(values)) | |
| .range(['#000', '#fff']); | |
| }, | |
| 'histogramEqualize': function(values){ | |
| var buckets = 100; | |
| var quantiles = d3.scale.quantile() | |
| .domain(values) | |
| .range(d3.range(buckets)) | |
| .quantiles(); | |
| var stopCount = quantiles.length; | |
| var linearScale = d3.scale.linear() | |
| .domain([0, stopCount - 1]) | |
| .range([d3.rgb('rgb(0, 0, 0)'), d3.rgb('rgb(255, 255, 255)')]); | |
| var grayScale = d3.range(stopCount).map(function(d){ | |
| return linearScale(d); | |
| }); | |
| return d3.scale.linear().domain(quantiles).range(grayScale); | |
| } | |
| }; | |
| var img = new Image; | |
| img.onload = function(){ | |
| cnvori.width = cnv.width = img.width; | |
| cnvori.height = cnv.height = img.height; | |
| ctx.drawImage(img, 0, 0, img.width, img.height); | |
| ctxori.drawImage(img, 0, 0, img.width, img.height); | |
| var imgData = ctx.getImageData(0, 0, img.width, img.height); | |
| var rasterData = []; | |
| for(j = 0; j < (imgData.data.length / 4); j++){ | |
| var brightness = d3.lab(d3.rgb(imgData.data[j * 4], | |
| imgData.data[j * 4 + 1], | |
| imgData.data[j * 4 + 2])).l; | |
| rasterData.push(imgData.data[j * 4] === 0 ? null : brightness); | |
| } | |
| var scale = colorScales.histogramEqualize(rasterData); | |
| for(j = 0; j < rasterData.length; j++){ | |
| var scaledColor = scale(rasterData[j]); | |
| var color = d3.rgb(scaledColor); | |
| imgData.data[j * 4] = color.r; | |
| imgData.data[j * 4 + 1] = color.g; | |
| imgData.data[j * 4 + 2] = color.b; | |
| imgData.data[j * 4 + 3] = 255; | |
| } | |
| ctx.putImageData(imgData, 0, 0); | |
| }; | |
| img.crossOrigin = ''; | |
| img.src = 'https://upload.wikimedia.org/wikipedia/commons/0/08/Unequalized_Hawkes_Bay_NZ.jpg'; | |
| </script> | |
| </body> |
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