You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
160 lines
4.2 KiB
160 lines
4.2 KiB
|
|
/**
|
|
* Associates a canvas to a given image, containing a number of renderings
|
|
* of the image at various sizes.
|
|
*
|
|
* This technique is known as 'mipmapping'.
|
|
*
|
|
* NOTE: Images can also be of type 'data:svg+xml`. This code also works
|
|
* for svg, but the mipmapping may not be necessary.
|
|
*/
|
|
class CachedImage {
|
|
constructor(image) {
|
|
this.NUM_ITERATIONS = 4; // Number of items in the coordinates array
|
|
|
|
this.image = new Image();
|
|
this.canvas = document.createElement('canvas');
|
|
}
|
|
|
|
|
|
/**
|
|
* Called when the image has been succesfully loaded.
|
|
*/
|
|
init() {
|
|
if (this.initialized()) return;
|
|
|
|
var w = this.image.width;
|
|
var h = this.image.height;
|
|
|
|
// Ease external access
|
|
this.width = w;
|
|
this.height = h;
|
|
|
|
// Make canvas as small as possible
|
|
this.canvas.width = 3*w/4;
|
|
this.canvas.height = h/2;
|
|
|
|
// Coordinates and sizes of images contained in the canvas
|
|
// Values per row: [top x, left y, width, height]
|
|
this.coordinates = [
|
|
[ 0 , 0 , w/2 , h/2],
|
|
[ w/2 , 0 , w/4 , h/4],
|
|
[ w/2 , h/4, w/8 , h/8],
|
|
[ 5*w/8, h/4, w/16, h/16]
|
|
];
|
|
|
|
this._fillMipMap();
|
|
}
|
|
|
|
|
|
/**
|
|
* @return {Boolean} true if init() has been called, false otherwise.
|
|
*/
|
|
initialized() {
|
|
return (this.coordinates !== undefined);
|
|
}
|
|
|
|
|
|
/**
|
|
* Redraw main image in various sizes to the context.
|
|
*
|
|
* The rationale behind this is to reduce artefacts due to interpolation
|
|
* at differing zoom levels.
|
|
*
|
|
* Source: http://stackoverflow.com/q/18761404/1223531
|
|
*
|
|
* This methods takes the resizing out of the drawing loop, in order to
|
|
* reduce performance overhead.
|
|
*
|
|
* @private
|
|
*/
|
|
_fillMipMap() {
|
|
var ctx = this.canvas.getContext('2d');
|
|
|
|
// First zoom-level comes from the image
|
|
var to = this.coordinates[0];
|
|
ctx.drawImage(this.image, to[0], to[1], to[2], to[3]);
|
|
|
|
// The rest are copy actions internal to the canvas/context
|
|
for (let iterations = 1; iterations < this.NUM_ITERATIONS; iterations++) {
|
|
let from = this.coordinates[iterations - 1];
|
|
let to = this.coordinates[iterations];
|
|
|
|
ctx.drawImage(this.canvas,
|
|
from[0], from[1], from[2], from[3],
|
|
to[0], to[1], to[2], to[3]
|
|
);
|
|
}
|
|
}
|
|
|
|
|
|
/**
|
|
* Draw the image, using the mipmap if necessary.
|
|
*
|
|
* MipMap is only used if param factor > 2; otherwise, original bitmap
|
|
* is resized. This is also used to skip mipmap usage, e.g. by setting factor = 1
|
|
*
|
|
* Credits to 'Alex de Mulder' for original implementation.
|
|
*
|
|
* ctx {Context} context on which to draw zoomed image
|
|
* factor {Float} scale factor at which to draw
|
|
*/
|
|
drawImageAtPosition(ctx, factor, left, top, width, height) {
|
|
if (factor > 2 && this.initialized()) {
|
|
// Determine which zoomed image to use
|
|
factor *= 0.5;
|
|
let iterations = 0;
|
|
while (factor > 2 && iterations < this.NUM_ITERATIONS) {
|
|
factor *= 0.5;
|
|
iterations += 1;
|
|
}
|
|
|
|
if (iterations >= this.NUM_ITERATIONS) {
|
|
iterations = this.NUM_ITERATIONS - 1;
|
|
}
|
|
//console.log("iterations: " + iterations);
|
|
|
|
let from = this.coordinates[iterations];
|
|
ctx.drawImage(this.canvas,
|
|
from[0], from[1], from[2], from[3],
|
|
left, top, width, height
|
|
);
|
|
} else if (this._isImageOk()) {
|
|
// Draw image directly
|
|
ctx.drawImage(this.image, left, top, width, height);
|
|
}
|
|
}
|
|
|
|
|
|
/**
|
|
* Check if image is loaded
|
|
*
|
|
* Source: http://stackoverflow.com/a/1977898/1223531
|
|
*
|
|
* @private
|
|
*/
|
|
_isImageOk(img) {
|
|
var img = this.image;
|
|
|
|
// During the onload event, IE correctly identifies any images that
|
|
// weren’t downloaded as not complete. Others should too. Gecko-based
|
|
// browsers act like NS4 in that they report this incorrectly.
|
|
if (!img.complete) {
|
|
return false;
|
|
}
|
|
|
|
// However, they do have two very useful properties: naturalWidth and
|
|
// naturalHeight. These give the true size of the image. If it failed
|
|
// to load, either of these should be zero.
|
|
|
|
if (typeof img.naturalWidth !== "undefined" && img.naturalWidth === 0) {
|
|
return false;
|
|
}
|
|
|
|
// No other way of checking: assume it’s ok.
|
|
return true;
|
|
}
|
|
}
|
|
|
|
|
|
export default CachedImage;
|