Prototyping image downscaling to a specific width in order to improve the thumbnail feature
Based on this StackOverflow answer
Built with blockbuilder.org
<!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; }
#downscaled {
position: absolute;
width: 230px;
top: 40px;
left: 10px;
}
#preview {
position: absolute;
left: 260px;
}
#file {
position: absolute;
top: 10px;
}
#dimensions {
position: absolute;
top: 130px;
}
textarea {
position:absolute;
top: 160px;
left: 0;
width: 200px;
height:340px;
}
</style>
</head>
<body>
<div id="downscaled"></div>
<img id="preview">
<input id="file" type="file" name="files[]">
<div id="dimensions"></div>
<textarea placeholder="json array output"></textarea>
<script>
var targetWidth = 72;
d3.select("#file").node()
.addEventListener("change", function(evt) {
d3.select("#downscaled").selectAll("canvas").remove();
var files = evt.target.files;
var file = files[0];
if(file.type.indexOf("image/") === 0) {
var reader = new FileReader();
reader.onload = (function(data) {
var img = document.getElementById('preview')
img.src = data.target.result;
setTimeout(function() {
scaleImage(img);
}, 300);
});
reader.readAsDataURL(file);
}
})
function scaleImage(img) {
var imgCV = document.createElement('canvas');
imgCV.width = img.width;
imgCV.height = img.height;
var imgCtx = imgCV.getContext('2d');
imgCtx.drawImage(img, 0, 0);
var scale = targetWidth / img.width;
console.log("scale", scale)
var downscaled = downScaleCanvas(imgCV, scale);
console.log("scaled", targetWidth, img.height * scale);
d3.select("#dimensions").text(targetWidth + " x " + img.height * scale);
d3.select("#downscaled").node()
.appendChild(downscaled.canvas);
d3.select("textarea").text(JSON.stringify(downscaled.jsonArray));
}
////stackoverflow.com/questions/18922880/html5-canvas-resize-downscale-image-high-quality
////jsfiddle.net/gamealchemist/r6aVp/
function downScaleCanvas(cv, scale) {
if (!(scale < 1) || !(scale > 0)) throw ('scale must be a positive number <1 ');
var sqScale = scale * scale; // square scale = area of source pixel within target
var sw = cv.width; // source image width
var sh = cv.height; // source image height
var tw = Math.floor(sw * scale); // target image width
var th = Math.floor(sh * scale); // target image height
var sx = 0, sy = 0, sIndex = 0; // source x,y, index within source array
var tx = 0, ty = 0, yIndex = 0, tIndex = 0; // target x,y, x,y index within target array
var tX = 0, tY = 0; // rounded tx, ty
var w = 0, nw = 0, wx = 0, nwx = 0, wy = 0, nwy = 0; // weight / next weight x / y
// weight is weight of current source point within target.
// next weight is weight of current source point within next target's point.
var crossX = false; // does scaled px cross its current px right border ?
var crossY = false; // does scaled px cross its current px bottom border ?
var sBuffer = cv.getContext('2d').
getImageData(0, 0, sw, sh).data; // source buffer 8 bit rgba
var tBuffer = new Float32Array(3 * tw * th); // target buffer Float32 rgb
var sR = 0, sG = 0, sB = 0; // source's current point r,g,b
/* untested !
var sA = 0; //source alpha */
for (sy = 0; sy < sh; sy++) {
ty = sy * scale; // y src position within target
tY = 0 | ty; // rounded : target pixel's y
yIndex = 3 * tY * tw; // line index within target array
crossY = (tY != (0 | ty + scale));
if (crossY) { // if pixel is crossing botton target pixel
wy = (tY + 1 - ty); // weight of point within target pixel
nwy = (ty + scale - tY - 1); // ... within y+1 target pixel
}
for (sx = 0; sx < sw; sx++, sIndex += 4) {
tx = sx * scale; // x src position within target
tX = 0 | tx; // rounded : target pixel's x
tIndex = yIndex + tX * 3; // target pixel index within target array
crossX = (tX != (0 | tx + scale));
if (crossX) { // if pixel is crossing target pixel's right
wx = (tX + 1 - tx); // weight of point within target pixel
nwx = (tx + scale - tX - 1); // ... within x+1 target pixel
}
sR = sBuffer[sIndex ]; // retrieving r,g,b for curr src px.
sG = sBuffer[sIndex + 1];
sB = sBuffer[sIndex + 2];
/* !! untested : handling alpha !!
sA = sBuffer[sIndex + 3];
if (!sA) continue;
if (sA != 0xFF) {
sR = (sR * sA) >> 8; // or use /256 instead ??
sG = (sG * sA) >> 8;
sB = (sB * sA) >> 8;
}
*/
if (!crossX && !crossY) { // pixel does not cross
// just add components weighted by squared scale.
tBuffer[tIndex ] += sR * sqScale;
tBuffer[tIndex + 1] += sG * sqScale;
tBuffer[tIndex + 2] += sB * sqScale;
} else if (crossX && !crossY) { // cross on X only
w = wx * scale;
// add weighted component for current px
tBuffer[tIndex ] += sR * w;
tBuffer[tIndex + 1] += sG * w;
tBuffer[tIndex + 2] += sB * w;
// add weighted component for next (tX+1) px
nw = nwx * scale
tBuffer[tIndex + 3] += sR * nw;
tBuffer[tIndex + 4] += sG * nw;
tBuffer[tIndex + 5] += sB * nw;
} else if (crossY && !crossX) { // cross on Y only
w = wy * scale;
// add weighted component for current px
tBuffer[tIndex ] += sR * w;
tBuffer[tIndex + 1] += sG * w;
tBuffer[tIndex + 2] += sB * w;
// add weighted component for next (tY+1) px
nw = nwy * scale
tBuffer[tIndex + 3 * tw ] += sR * nw;
tBuffer[tIndex + 3 * tw + 1] += sG * nw;
tBuffer[tIndex + 3 * tw + 2] += sB * nw;
} else { // crosses both x and y : four target points involved
// add weighted component for current px
w = wx * wy;
tBuffer[tIndex ] += sR * w;
tBuffer[tIndex + 1] += sG * w;
tBuffer[tIndex + 2] += sB * w;
// for tX + 1; tY px
nw = nwx * wy;
tBuffer[tIndex + 3] += sR * nw;
tBuffer[tIndex + 4] += sG * nw;
tBuffer[tIndex + 5] += sB * nw;
// for tX ; tY + 1 px
nw = wx * nwy;
tBuffer[tIndex + 3 * tw ] += sR * nw;
tBuffer[tIndex + 3 * tw + 1] += sG * nw;
tBuffer[tIndex + 3 * tw + 2] += sB * nw;
// for tX + 1 ; tY +1 px
nw = nwx * nwy;
tBuffer[tIndex + 3 * tw + 3] += sR * nw;
tBuffer[tIndex + 3 * tw + 4] += sG * nw;
tBuffer[tIndex + 3 * tw + 5] += sB * nw;
}
} // end for sx
} // end for sy
// create result canvas
var resCV = document.createElement('canvas');
resCV.width = tw;
resCV.height = th;
var resCtx = resCV.getContext('2d');
var imgRes = resCtx.getImageData(0, 0, tw, th);
var jsonArray = [];
var tByteBuffer = imgRes.data;
// convert float32 array into a UInt8Clamped Array
var pxIndex = 0; //
var r,g,b;
for (sIndex = 0, tIndex = 0; pxIndex < tw * th; sIndex += 3, tIndex += 4, pxIndex++) {
r = tByteBuffer[tIndex] = Math.ceil(tBuffer[sIndex]);
g = tByteBuffer[tIndex + 1] = Math.ceil(tBuffer[sIndex + 1]);
b = tByteBuffer[tIndex + 2] = Math.ceil(tBuffer[sIndex + 2]);
tByteBuffer[tIndex + 3] = 255;
jsonArray.push([r,g,b]);
}
// writing result to canvas.
resCtx.putImageData(imgRes, 0, 0);
return { canvas: resCV, jsonArray: jsonArray };
}
</script>
</body>