MS RFC 113: Layer Compositing Pipeline

Author:Thomas Bonfort
Version:MapServer 7.0

1. Motivation

Some cartographic renderings would benefit from the addition of advanced blending modes, as can be explained in detail in . In particular, this functionality is essential for more pleasant renderings of raster hillshadings over vector surfaces.

2. Proposed enhancement

The LAYER->OPACITY functionality will be replaced with one or more LAYER->COMPOSITE blocks that will define which compositing mode should be used to blend the current layer onto the pre-exisiting basemap. Similarly to when using the actual LAYER->OPACITY mechanism, the addition of a LAYER->COMPOSITE block will instruct mapserver to render the current layer in a temporary image buffer and composite it back onto the base image in a final step.

    COMPOP "multiply"
    OPACITY 70
    COMPFILTER "foo" #reserved for future use

The COMPFILTER keyword will allow to also apply some kind of filter to the pixels before compositing them onto the base image (this would typically be used to apply a blurring filter to the layer to obtain a halo effect around geometries). The syntax and implementations for such features will be determined in a future release and RFC. Supporting multiple COMPOSITE blocks inside a LAYER will only make real sense once these COMPFILTERs are decided upon.

2.1 Supported Compositing Operations

The default compositing operator which is equivalent to the current layer compositing operator is “src-over”. The COMPOP values that can be used should be explicit and are listed here:

  • clear
  • color-burn
  • color-dodge
  • contrast*
  • darken
  • difference
  • dst
  • dst-atop
  • dst-in
  • dst-out
  • dst-over
  • exclusion
  • hard-light
  • invert*
  • invert-rgb*
  • lighten
  • minus*
  • multiply
  • overlay
  • plus
  • screen
  • soft-light
  • src
  • src-atop
  • src-in
  • src-out
  • src-over
  • xor

Operators marked with a star (*) will only be supported when using an AGG backends and when pixman support is not enabled, and will fall back to “src-over” when this is not the case.

2.2 Backwards Compatibility

LAYER->OPACITY will be deprecated and should be replaced by

    OPACITY 70

in new mapfiles. The mapfile parser and mapscript setters/getters will be updated to work with LAYER->OPACITY but will produce unspecified results if used in conjunction with COMPOSITE blocks.

2.3 Performance Implications

None if not explicitly enabled. Depending on the selected compositing operator performance may be slightly degraded compared to when using “src-over”.

2.4 Limitations

  • The Compositing pipeline is ony supported by pixel renderers, and as such will not be functional for PDF and SVG backends.
  • Some compositing operators are not supported by the cairo backends or when pixman support has been enabled.

3. Implementation Details

3.1 Architecture

  • Renderers will have their vtable signature updated to include a compositeRasterBuffer() function to be used when compositing has been requested.
  • msDrawMap() will be modified to create a temporary image and then call compositeRasterBuffer() once the layer rendering is over.
  • TBD: The current implementation can be configured to use pixman as an external dependency to enable use of SSE optimized operations inside the AGG renderer. Whether this will be kept in the long term should be decided upon depending on the cost/benefit ratio (performance, features, dependencies)

3.1 Affected files

  • mapfile.c, maplexer.l: parser, keywords.
  • mapagg.cpp, mapcairo.c: actual compositing operations
  • mapcontext.c mapcopy.c, mapkml.c, maplegend.c, mapogcsld.c, maputil.c, mapscripts: housekeeping and backwards compatibility
  • mapdraw.c: calls to compositing backends on drawing operations

3.2 Tracking Issue

4. Voting History

+1 from ThomasB, StephanM, MikeS, SteveW, DanielM, TomK and PerryN