Function constructImageData

extract the ImageData from an image source (of type CanvasImageSource), with optional cropping.

due to the fact that this function utilizes a Canvas/OffscreenCanvas, it is important to note that the output ImageData is sometimes lossy in nature. this is because gpu-accelerated web-browsers approximate the colors (i.e. you don't truly get 256^3 unique colors), and also due to rounding errors from/to internal float-valued colors and output integer-valued colors.

but generally speaking, the ImageData can be lossless if all of the following are satisfied:

• disable gpu-acceleration of your web-browser, through the flags page ("chrome://flags" or "about:config").
• your img_src has either no alpha-channel, or 100% visible alpha-channel throughout (i.e. non-transparent image).
• you have pre-multiplied alpha disabled (TODO: this part can be achieved/implemented by this library, but I haven't looked into it yet.). check out this script for using webgl for reading and writing bitmap pixels without losing color accuracy due to alpha premultiplication