How to clear the canvas for redrawing
After experimenting with composite operations and drawing images on the canvas I'm now trying to remove images and compositing. How do I do this?
I need to clear the canvas for redrawing other images; this can go on for a while so I don't think drawing a new rectangle every time will be the most efficient option.
Verified Answer (1261 Votes) ✓
const context = canvas.getContext('2d'); context.clearRect(0, 0, canvas.width, canvas.height);
Answer #2 (713 Votes)
context.clearRect(0, 0, canvas.width, canvas.height);
This is the fastest and most descriptive way to clear the entire canvas.
Do not use:
canvas.width = canvas.width;
canvas.width resets all canvas state (e.g. transformations, lineWidth, strokeStyle, etc.), it is very slow (compared to clearRect), it doesn't work in all browsers, and it doesn't describe what you are actually trying to do.
Dealing with transformed coordinates
If you have modified the transformation matrix (e.g. using
context.clearRect(0,0,canvas.width,canvas.height) will likely not clear the entire visible portion of the canvas.
The solution? Reset the transformation matrix prior to clearing the canvas:
// Store the current transformation matrix context.save(); // Use the identity matrix while clearing the canvas context.setTransform(1, 0, 0, 1, 0, 0); context.clearRect(0, 0, canvas.width, canvas.height); // Restore the transform context.restore();
Edit: I've just done some profiling and (in Chrome) it is about 10% faster to clear a 300x150 (default size) canvas without resetting the transform. As the size of your canvas increases this difference drops.
That is already relatively insignificant, but in most cases you will be drawing considerably more than you are clearing and I believe this performance difference be irrelevant.
100000 iterations averaged 10 times: 1885ms to clear 2112ms to reset and clear
Answer #3 (220 Votes)
If you are drawing lines, make sure you don't forget:
Otherwise the lines won't get cleared.