Moving already Drawn shape on html5 canvas - html

Question

I'm having trouble keeping one object on my canvas. The initially drawn box rendered it in the correct position, but it disappears when I drag it:
var canvas = document.getElementById('canvas'),
context = canvas.getContext('2d'),
eraseAllButton = document.getElementById('eraseAllButton'),
strokeStyleSelect = document.getElementById('strokeStyleSelect'),
guidewireCheckbox = document.getElementById('guidewireCheckbox'),
drawingSurfaceImageData,
mousedown = {},
rubberbandRect = {},
dragging = false,
guidewires = guidewireCheckbox.checked,
w = 90, h = 90;
count = 0;
boxesXCo = 0;
boxesYCo = 0;
i = 0;
// Functions..........................................................
function drawGrid(color, stepx, stepy) {
context.save()
context.strokeStyle = color;
context.lineWidth = 0.5;
context.clearRect(0, 0, context.canvas.width, context.canvas.height);
for (var i = stepx + 0.5; i < context.canvas.width; i += stepx) {
context.beginPath();
context.moveTo(i, 0);
context.lineTo(i, context.canvas.height);
context.stroke();
}
for (var i = stepy + 0.5; i < context.canvas.height; i += stepy) {
context.beginPath();
context.moveTo(0, i);
context.lineTo(context.canvas.width, i);
context.stroke();
}
context.restore();
}
function windowToCanvas(x, y) {
var bbox = canvas.getBoundingClientRect();
return { x: x - bbox.left * (canvas.width / bbox.width),
y: y - bbox.top * (canvas.height / bbox.height) };
}
// Save and restore drawing surface...................................
function saveDrawingSurface() {
drawingSurfaceImageData = context.getImageData(0, 0,
canvas.width,
canvas.height);
}
function restoreDrawingSurface() {
context.putImageData(drawingSurfaceImageData, 0, 0);
}
function drawRubberbandShape(loc) {
context.beginPath();
context.moveTo(mousedown.x, mousedown.y);
//context.lineTo(loc.x, loc.y);
context.stroke();
}
function updateRubberband(loc) {
//updateRubberbandRectangle(loc);
context.restore();
drawRubberbandShape(loc);
}
// Guidewires.........................................................
function drawHorizontalLine (y) {
context.beginPath();
context.moveTo(0,y+0.5);
context.lineTo(context.canvas.width,y+0.5);
context.stroke();
}
function drawVerticalLine (x) {
context.beginPath();
context.moveTo(x+0.5,0);
context.lineTo(x+0.5,context.canvas.height);
context.stroke();
}
function drawGuidewires(x, y) {
context.save();
context.strokeStyle = 'rgba(0,0,230,0.4)';
context.lineWidth = 0.5;
drawVerticalLine(x);
drawHorizontalLine(y);
context.restore();
}
// Canvas event handlers..............................................
canvas.onmousedown = function (e) {
var loc = windowToCanvas(e.clientX, e.clientY);
e.preventDefault(); // prevent cursor change
context.restore();
saveDrawingSurface();
mousedown.x = loc.x;
mousedown.y = loc.y;
dragging = true;
if (i ==0)
i++;
else if(((mousedown.x<=(boxesXCo+w)&&(mousedown.x>=boxesXCo))&&
((mousedown.y<=(boxesYCo+h)&&(mousedown.y>=boxesYCo)))))
i--;
};
canvas.onmousemove = function (e) {
var loc;
if (dragging) {
e.preventDefault(); // prevent selections
loc = windowToCanvas(e.clientX, e.clientY);
restoreDrawingSurface();
//updateRubberband(loc);
if(guidewires) {
drawGuidewires(loc.x, loc.y);
}
}
if(((mousedown.x<=(boxesXCo+w)&&(mousedown.x>=boxesXCo))&&
((mousedown.y<=(boxesYCo+h)&&(mousedown.y>=boxesYCo))))
&& (dragging)&&(i == 1 )){
context.restore();
restoreDrawingSurface();
context.fillStyle = strokeStyleSelect.value;
context.fillRect(e.clientX,e.clientY,w,h);
};
//Trying to implement moving shapes but need to store values of drawn objs
};
canvas.onmouseup = function (e) {
loc = windowToCanvas(e.clientX, e.clientY);
restoreDrawingSurface();
updateRubberband(loc);
dragging = false;
if(i == 0);
else {
saveDrawingSurface();
restoreDrawingSurface();
context.fillRect(e.clientX,e.clientY, w, h);
boxesXCo = e.clientX;
boxesYCo = e.clientY;
context.restore();
i++;}
/*else if(i == 1)
{
context.restore();
}*/
//context.fillRect(mousedown.x,mousedown.y,w,h,"FF0982");
};
// Controls event handlers.......................................
eraseAllButton.onclick = function (e) {
context.clearRect(0, 0, canvas.width, canvas.height);
drawGrid('lightgray', 10, 10);
saveDrawingSurface();
count =0;
context.restore();
};
strokeStyleSelect.onchange = function (e) {
context.strokeStyle = strokeStyleSelect.value;
context.fillStyle = strokeStyleSelect.value;
};
guidewireCheckbox.onchange = function (e) {
guidewires = guidewireCheckbox.checked;
};
// Initialization................................................
context.strokeStyle = strokeStyleSelect.value;
context.fillStyle = strokeStyleSelect.value;
drawGrid('lightgray', 10, 10);
//context.fillRect(mousedown.x,mousedown.y,(mousedown.x+50),(mousedown.x+50),"FF0982");
//context.drawRect(mousedown.x-50,mousedown.y-50,mousedown.x+50,mousedown.y+50);
thanx again

Answer 1

Rather than drawing to canvas on each mouse movement, use window.requestFrameAnimation to draw constantly.
At a guess, I'd assume either the browser rendering is interfering or the coordinates are out but I can't run your code to make sure.
I've written some pseudo code that shows what I do for rendering things. You'll need to implement this for yourself, it won't run out of the box, it's just intended as a guide.
var Box = function() {
};
Box.prototype = {
x: 0,
y: 0,
draw: function (canvas) {
// draw the item on the canvas
}
};
var box = new Box();
window.requestAnimationFrame(function() {
box.draw(canvas);
window.requestAnimationFrame();
});
canvas.onmousemove = function(e) {
if (dragging) {
box.x = e.clientX;
box.y = e.clientY;
}
}