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Per-layer opacity and compositing #314

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artemp opened this Issue Oct 11, 2011 · 6 comments

4 participants

@artemp
Mapnik member
artemp commented Oct 11, 2011

Let's say I want to render a semi-transparent street overlay to render on top of an aerial image, and that I want to have the following layers:

  • Aerial image
  • Street overlay
  • Labels and point symbols

I would like the street overlay layer to be semi-transparent. If the individual symbolizers are semi-transparent, lots of rendering artifacts will appear:

  • Road casing and road fill will clash - the road casing will be visible through the road fill, and the road fill will be more opaque than the border.
  • Crossroads will be more opaque than the individual roads, as all roads will be rendered on top of each other.

Thus, the solution to this problem would be to have two new parameters for every layer: opacity and compositing mode.

Every layer should get its own canvas; once all elements in a layer have been rendered, that layer canvas is composed on top of the canvas for the resulting image.

For bonus points, instantiate canvases only for layers with 1.0 opacity and normal compositing, in order to not waste memory.

@chngl
chngl commented Jan 27, 2012

Hope it can be solved soon

@springmeyer
Mapnik member

/cc @ivansanchez and @novldp - who were the original reporters of this bug back when we used trac.

@springmeyer springmeyer was assigned Jun 19, 2012
@springmeyer
Mapnik member

new work I'm about to commit implements this, adding a new style-level opacity value:

Whereas this is the same map with the same opacity value, but at the symbolizer level (the only previous option in older mapnik):

@springmeyer
Mapnik member

should have mentioned that comp-op is now also supported at style level (as well as symbolizer level), and will be available in the upcoming 2.1.0 release.

@novldp
novldp commented Jun 19, 2012

Awesome progress! This will indeed make life easy for these types of renders.

@springmeyer
Mapnik member

great to hear @novldp :)

@springmeyer springmeyer added a commit that referenced this issue Jun 21, 2012
@springmeyer springmeyer use the current buffer in markers, building, shield, and line pattern…
… symbolizers to ensure that style level opacity and compositing work - refs #1262 and #314
af519a4
@springmeyer springmeyer added a commit that referenced this issue Jun 21, 2012
@springmeyer springmeyer fix behavior of compositing via python api now that we have premultip…
…lication issues sorted out in core - refs #1262 and #314 and #1211
eb4740b
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