Changes between Initial Version and Version 1 of GSoC_SVG_Symbols_SECOND_DRAFT_OF_THE_MS_RFC

Timestamp:

Aug 10, 2009, 10:45:40 AM (15 years ago)

Author:

kiranvarma

Comment:

--

GSoC_SVG_Symbols_SECOND_DRAFT_OF_THE_MS_RFC

@@ +1 @@
+                                MS RFC – SVG symbol Support with AGG
+                                   Kiran Anjaneya Varma Alluri
+                                    August 10, 2009
+1 Overview
+Popular tools such as Inkscape generate SVG graphics natively. So far MapServer has not supported the SVG format, which
+leaves many users with the extra work of converting between formats. This RFC aims to introduce support for SVG graphics
+as an alternate method to define vector symbols in the MapServer symbolset.
+
+2 Method
+The SVG parsing and rendering will be triggered by the following syntax:
+SYMBOL
+TYPE SVG
+SVGPATH / p a t h / t o / f i l e . svg
+END
+The choice for using a SYMBOL for the task is to allow for the reuse of the SVG symbol easily within classes.
+
+3 Technical Overview
+In order to support the SVG format, we make use of the AGG library. The reasons for going with the AGG library are:
+1. It has already been integrated into MapServer. While other popular libraries, such as Cairo, can be used, they would
+require effort to integrate into MapServer and would further introduce many dependencies into the MapServer code.
+2. It is a very lightweight library and delivers performance equal to, and sometimes higher than other popular libraries.
+
+There are some disadvantages that emerge from using the AGG library. Foremost, the AGG library has an incomplete SVG
+specification. To avoid having to reimplement the missing subset of the specification (which would, without a doubt, be a
+non-trivial task, and would add to the complexity of this RFC), we stick by the following techniques:
+
+1. A modular design is used, where the generation of the intermediate pixmap is done by a single function so that it can be
+easily modified to adjust to a diferent library. Thus, using a pixmap will allow us to use any rendering library.
+1
+2. The unimplemented subset of the SVG specification is clearly provided to users in the documentation to prevent frustration
+on their side.
+
+
+To implement this functionality, the following steps needs to be accomplished:
+1. Implement an SVG parser. For this, we directly make use of the SVG Viewer example provided along with the AGG
+sources.
+2. Modify the MapServer lexical analyzer and parser to recognize the new keywords (SVG and SVGPATH).
+3. Modify the symbolObj structure to store information about the SVG symbol requested (eg. File path).
+4. Write a single function to render a parsed SVG file to a pixmap. The pixmap will be an intermediate form that can then
+be rendered. The pixmap will be stored in cache memory to prevent having to constantly be rasterized it every time a
+symbol is drawn.
+5. Create a createBrushFromSVG() function that creates a GD image pixmap. Also modify the basic rendering functions
+to support the new symbol type by using the createBrushFromSVG function.
+6. Render the pixmap into the final image file. This will be done using a mixture of the AGG and GD libraries.
+
+
+4 Files Added and Modified
+4.1 AGG/SVG Parser
+The SVG parsing and pixmap generation functions will be stored in source files stored at
+/ maps e rve r / r e n d e r e r s / agg / svg /
+The files within the above directory include the files for:
+a. SVG Parsing
+b. Trigger Function (based on Mapfile syntax)
+c. SVG to Pixmap generation
+All files are within the C++ ’mapserver’ namespace.
+
+4.2 Rendering Functions
+The rendering functions in mapgd.c will be modified to support the SVG type. These include all the basic shapes such as
+points, lines, circles etc. These will make use of the createBrushFromSVG function to render to a GD pixmap.
+
+4.3 Lexical Analyzer and Parser
+The maplexer.l file will be modified to support two new keywords (SVG and SVGPATH). The mapsymbol.h file be modified
+by adding SVG related data to the symbolObj structure along with basic enumerations and defines. The mapsymbol.c file’s
+loadSymbol function will be modified to generate the required new fields in the symbolObj structure.
+
+
+5 Documentation
+The following documentation needs to be prepared for this project:
+1. Missing SVG specification document.
+2. Basic usage tutorial of new functionality with examples.