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Fix download images to match live gl3d plot #4566

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merged 5 commits into from
Mar 10, 2020
Merged

Fix download images to match live gl3d plot #4566

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0e5c9c5
use temporary variables for swap indices
archmoj Feb 10, 2020
41c7783
move flip pixel logic into a function
archmoj Feb 10, 2020
a259945
rename loop vars to i, j, k
archmoj Feb 10, 2020
6878635
correct RGB - no need to multiply alpha
archmoj Feb 10, 2020
2244c5e
update gl3d baselines - now match with viewport display
archmoj Feb 10, 2020
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18 changes: 18 additions & 0 deletions src/plots/gl3d/scene.js
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Original file line number Diff line number Diff line change
Expand Up @@ -1003,6 +1003,23 @@ function flipPixels(pixels, w, h) {
}
}

function correctRGB(pixels, w, h) {
for(var i = 0; i < h; ++i) {
for(var j = 0; j < w; ++j) {
var k = 4 * (w * i + j);

var a = pixels[k + 3]; // alpha
if(a > 0) {
var q = 255 / a;

for(var l = 0; l < 3; ++l) { // RGB
pixels[k + l] = Math.min(q * pixels[k + l], 255);
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Can explain why we need to apply this transform when exporting images?

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Unlike openGL, webGL premultipliedAlpha in enabled by default.
To get correct RGB values from the webgl context it was needed to devide RGB values by alpha.

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Ok, what would happen if we instead disable premultipliedAlpha on image exports?

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Good question; but that didn't work: demo.

}
}
}
}
}

proto.toImage = function(format) {
var scene = this;

Expand All @@ -1022,6 +1039,7 @@ proto.toImage = function(format) {
var pixels = new Uint8Array(w * h * 4);
gl.readPixels(0, 0, w, h, gl.RGBA, gl.UNSIGNED_BYTE, pixels);
flipPixels(pixels, w, h);
correctRGB(pixels, w, h);

var canvas = document.createElement('canvas');
canvas.width = w;
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