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4_12_modify_image_data.html
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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Modify ImageData</title>
</head>
<body>
<button id="resetButton">Reset</button><br>
<canvas id="canvas" width="600" height="375"></canvas>
</body>
<script type="text/javascript">
var canvas = document.getElementById('canvas'),
context = canvas.getContext('2d'),
resetButton = document.getElementById('resetButton'),
image = new Image(),
imageData,
imageDataCopy = context.createImageData(canvas.width, canvas.height),
mousedown = {},
rubberbandRectangle = {},
dragging = false;
function windowToCanvas (canvas, x, y) {
var canvasRectangle = canvas.getBoundingClientRect();
return {
x : x - canvasRectangle.left,
y : y - canvasRectangle.top
};
}
function copyCanvasPixels () {
var i = 0;
for(i = 0; i < 3; i++) {
imageDataCopy.data[i] = imageData.data[i];
}
for(i = 3; i < imageData.data.length; i+=4) {
imageDataCopy.data[i] = imageData.data[i] / 2;
imageDataCopy.data[i+1] = imageData.data[i+1];
imageDataCopy.data[i+2] = imageData.data[i+2];
imageDataCopy.data[i+3] = imageData.data[i+3];
}
}
function captureRubberbandPixels () {
imageData = context.getImageData(0, 0, canvas.width, canvas.height);
copyCanvasPixels();
}
function restoreRubberbandPixels () {
var deviceWidthOverCssPixels = imageData.width / canvas.width,
deviceHeightOverCssPixels = imageData.height / canvas.height;
context.putImageData(imageData, 0, 0);
context.putImageData(imageDataCopy, 0, 0,
rubberbandRectangle.left + context.lineWidth,
rubberbandRectangle.top + context.lineWidth,
(rubberbandRectangle.width - 2*context.lineWidth) * deviceWidthOverCssPixels,
(rubberbandRectangle.height - 2*context.lineWidth) * deviceHeightOverCssPixels
);
}
function drawRubberband () {
context.strokeRect(
rubberbandRectangle.left + context.lineWidth,
rubberbandRectangle.top + context.lineWidth,
rubberbandRectangle.width - 2*context.lineWidth,
rubberbandRectangle.height - 2*context.lineWidth
);
}
function setRubberbandRectangle (x, y) {
rubberbandRectangle.left = Math.min(x, mousedown.x);
rubberbandRectangle.top = Math.min(y, mousedown.y);
rubberbandRectangle.width = Math.abs(x - mousedown.x);
rubberbandRectangle.height = Math.abs(y - mousedown.y);
}
function rubberbansStart (x, y) {
mousedown.x = x;
mousedown.y = y;
rubberbandRectangle.left = x;
rubberbandRectangle.top = y;
rubberbandRectangle.width = 0;
rubberbandRectangle.height = 0;
dragging = true;
captureRubberbandPixels();
}
function rubberbandStretch (x, y) {
if(rubberbandRectangle.width > 2*context.lineWidth && rubberbandRectangle.height > 2*context.lineWidth) {
if(imageData !== undefined) {
restoreRubberbandPixels();
}
}
setRubberbandRectangle(x, y);
if(rubberbandRectangle.width > 2*context.lineWidth && rubberbandRectangle.height > 2*context.lineWidth) {
drawRubberband();
}
}
function rubberbandEnd () {
context.putImageData(imageData, 0, 0);
context.drawImage(
canvas,
rubberbandRectangle.left + 2*context.lineWidth,
rubberbandRectangle.top + 2*context.lineWidth,
rubberbandRectangle.width - 4*context.lineWidth,
rubberbandRectangle.height - 4*context.lineWidth,
0, 0, canvas.width, canvas.height
);
dragging = false;
imageData = undefined;
}
canvas.onmousedown = function(e) {
var loc = windowToCanvas(canvas, e.clientX, e.clientY);
e.preventDefault();
rubberbansStart(loc.x, loc.y);
};
canvas.onmousemove = function(e) {
var loc;
if(dragging) {
loc = windowToCanvas(canvas, e.clientX, e.clientY);
rubberbandStretch(loc.x, loc.y);
}
};
canvas.onmouseup = function(e) {
rubberbandEnd();
};
// initialization
image.src = 'images/monster_in_love.jpg';
image.onload = function() {
context.drawImage(image, 0, 0, canvas.width, canvas.height);
};
resetButton.onclick = function() {
context.clearRect(0, 0, canvas.width, canvas.height);
context.drawImage(image, 0, 0, canvas.width, canvas.height);
};
context.strokeStyle = 'navy';
context.lineWidth = 1.0;
</script>
</html>