Clip layer rendering to limited extent

[tschaub]

For layers with limited extents, a clipping mask is set on the map canvas before drawing the layer canvas. This avoids composing unnecessary blank pixels and removes the need to clear dirty portions of the layer canvas outside the current layer extent.

FIxes #2920.

[tschaub]

There are a few optimizations that could be made here:

• when rotation is zero, rect could be used instead of moveTo and lineTo
• when clipping, restoring context.globalAlpha and resetting the transform (for non-zero rotation) could be avoided, since we already have to call context.restore()
• if the frame extent is contained within the layer extent, no clipping needs to be done

I'm not sure how much value there is in adding these optimizations. The first two in particular would make for a much bigger/clunkier composeFrame method. I'm interested to hear others' thoughts.

This doesn't change the behavior of the WebGL renderer with respect to limited layer extents (see #2434).

[tschaub]

Any chance for a review on this? I'm happy with the changes to both the Canvas layer renderer and tile layer renderer. And it fixes the rendering issue reported in #2920.

[elemoine]

Sorry Tim. Was very busy last week. I'll do the review tomorrow.

[ahocevar]

At least to me this looks good. Also nice to see that pre-/postcompose handlers can still use the full extent and are not affected by the clipping.

[elemoine]

Also nice to see that pre-/postcompose handlers can still use the full extent and are not affected by the clipping.

What do you mean exactly?

[ahocevar]

What do you mean exactly?

Even if the layer is clipped, I can draw on the whole viewport in layer pre- and postcompose handlers using the immediate API.

[elemoine]

@tschaub do vector layers get clipped as well?

Also, I regret that the WebGL renderer does not behave the same way. But that's ok.

Please merge when you think it's ready.

[tschaub]

@tschaub do vector layers get clipped as well?

Since vector layers don't accept an extent option, I'm not going to address that here. #2972 suggests making extent a bit more consistent between vector and raster layers.

#2434 is about WebGL renderer behavior.