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Last active March 17, 2017 02:19
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Reprojected Raster Clipping

This bl.ock takes an alternative approach to this one, in that it is not dependent on a circular projection disc.

It runs each pixel through a forward then inverse projection to see if the pixel should be plotted. It is in response to a SO question, [this was a potential answer] (http://stackoverflow.com/a/41856996/7106086). This is required in transforming the equiratangular source image as the d3 projection function is not one to one.

<!DOCTYPE html>
<meta charset="utf-8">
<style>
body {
background: #222;
}
</style>
<body>
<script src="https://d3js.org/d3.v3.min.js"></script>
<script src="https://d3js.org/d3.geo.projection.v0.min.js"></script>
<script>
var width = 960,
height = 500;
/*
var projection = d3.geoAzimuthalEquidistant()
.scale(65)
.precision(.1);
*/
var projection = d3.geo.projection(d3.geo.hammer.raw(1.75, 2))
.rotate([-10, -45])
.scale(130)
.translate([width / 2, height / 2])
.precision(.1);
var canvas = d3.select("body").append("canvas")
.attr("width", width)
.attr("height", height);
var context = canvas.node().getContext("2d");
var image = new Image;
image.onload = onload;
image.src = "readme-blue-marble.jpg";
function onload() {
var dx = image.width,
dy = image.height;
context.drawImage(image, 0, 0, dx, dy);
var sourceData = context.getImageData(0, 0, dx, dy).data,
target = context.createImageData(width, height),
targetData = target.data;
for (var y = 0, i = -1; y < height; ++y) {
for (var x = 0; x < width; ++x) {
var p = projection.invert([x, y]), λ = p[0], φ = p[1];
var pxy = projection(p);
var tolerance = 0.5;
if ( λ > 180 || λ < -180 || φ > 90 || φ < -90 ) { i += 4; continue; }
if ( (Math.abs(pxy[0] - x) < tolerance ) && (Math.abs(pxy[1] - y) < tolerance ) ) {
var q = ((90 - φ) / 180 * dy | 0) * dx + ((180 + λ) / 360 * dx | 0) << 2;
targetData[++i] = sourceData[q];
targetData[++i] = sourceData[++q];
targetData[++i] = sourceData[++q];
targetData[++i] = 255;
}
else {
i += 4;
}
}
}
context.clearRect(0, 0, width, height);
context.putImageData(target, 0, 0);
}
</script>
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