Zooming into canvas via js causes content to be lost? - html5-canvas

Question

http://deepschool.kd.io/Pages/Experiments/draw.htm
This is a Image editor I am working on, But it has a bug. Let's say you have created an image a 15x Zoom, when you change the zoom, the image is lost. Why is this? and what is the Remedy?
HTML:
Zoom:
<input type="number" id="zoom" min="1" max="50" onchange="zoomy()">
JS:
var zoomy = function() {
var zoomamount = zoom.value
var canvassize = zoomamount * 16
c.width = canvassize
c.height = canvassize
};
Thanks In Advance

Answer 1

If you want to zoom into canvas, it means you have to redraw it with zoom.
So instead of drawing pixels on click right onto canvas, which is made of pure pixels... You need to first create some representation of your grid of pixels.
var gridOfPixels = [];
Let's say you are ok with static size for now. Make it 8x8 pixels. At start you want to initialize your array:
for (var i=0; i < 8*8; i++) gridOfPixels[i] = 0;
So the grid canvas is ready, now we need to draw it.
function renderGrid() {
for (var y=0; y < 8; y++)
for (var x=0; x < 8; x++)
renderPixel( x, y, gridOfPixels[x+y*8] );
}
You already know how to renderPixel - calculate the rectangle position (posX = x*pixWidth, posY*pixHeight), where pixWidth is canvasWidth/8, etc.. Now you draw all your pixels, using the third parameter for the color.
To finish, you have to connect onclick to put a pixel on grid, and then call renderGrid so the user sees the change.
$('#my-canvas').click(function(e) {
var x = ...;
var y = ...; // calculate the position of pixels from mouse position inside canvas
// dont forget to check that x,y are in the 0-7 range
// dont forget to convert x,y to whole number using parseInt()
gridOfPixels[x+y*8] = 1;
renderGrid(); // update the grid canvas
});
Now, every time you resize the canvas or change some variables, the original canvas content will be saved in your grid, and you can renderGrid() any time you need to. You could even do it in realtime, animating the color of the pixels, etc..
Have fun. :)