Saint's Log

Implementing canvas putImageData’s optional arguments

Bug 498826 is about the HTML canvas putImageData method. It did not implement the optional arguments specified in the WHATWG spec. These optional arguments specify the dirty rectangle for the image data transfer (specifically, these arguments are the coordinates of the top left corner and the dimensions of the dirty rectangle, any of which are allowed to be negative). A quick glance at the description of the algorithm for handling of the optional arguments may not reveal the overall intent of the algorithm. Some of its key aspects are:

1. Adjusting the dimensions of the rectangle to be positive by shifting the top left corner if necessary (step 2).
2. Ensuring the top left corner of the dirty rectangle is in the first quadrant (step 3) which effectively eliminates all negative arguments.
3. Clipping the dirty rectangle to ensure its lower right corner does not extend beyond the bounds of the incoming imagedata’s dimensions (step 4).
4. Verifying that the newly adjusted dirty rectangle has positive dimensions (step 5), and if so, using the region bounded by the dirty rectangle on the incoming imagedata object as the source for the operation.

The patch is rather straightforward (although admittedly, it was not as straightforward to create on my part). If there are enough arguments to specify a dirty rectangle, then the JS_ValueToECMAInt32 function is used to convert the JavaScript values into integers. The CheckedInt32 and the gfxRect classes do most of the heavy lifting in the patch, and then only the dirty rectangle is redrawn.