putImageData(), how to keep old pixels if new pixels are transparent?

Question

In html5, when you draw to a canvas using putImageData(), if some of the pixels you are drawing are transparent (or semi-transparent), how do you keep old pixels in the canv

Answer 1

Something that tripped me up that may be of use... I had problems with this because I assumed that putImageData() and drawImage() would work in the same way but it seems they don't. putImageData() will overwrite existing pixels with its own transparent data while drawImage() will leave them untouched.

When looking into this I just glanced at the docs for CanvasRenderingContext2D.globalCompositeOperation (should have read more closely), saw that source-over is the default and didn't realise this would not apply to putImageData()

Drawing into a temporary canvas then and using drawImage() to add the temp canvas to the main context was the solution I needed so cheers for that.