[mapguide-internals] Trying to improve AGG

[Walt Welton-Lair]

Hi All,

I've attached a zip file to RFC 52 (https://trac.osgeo.org/mapguide/wiki/MapGuideRfc52) which includes some images showing what I think is an improvement in AGG rendering quality.  Basically I wanted to understand why AGG renders some lines thicker than expected.  This was reported on Zac's website: http://zacster.blogspot.com/2008/02/mapguide-20-agg-versus-gd-renderers.html.

Here's a summary of what I was experimenting with:

1/ I tested using two data sets: one containing relative sparse data and the other dense data.  The sparse data set was rendered using a hatch fill based on the new stylization, while the dense data set was rendered using just an outline.

2/ AGG does sub-pixel rendering.  While this lets it do nice anti-aliasing, it does result in some oddities like vertical / horizontal lines being thicker than expected (see the feedback on Zac's website).  I found that by clamping the point coordinates passed to AGG to integer values these oddities went away, but at the cost of slight pixelation.

3/ We currently do not generalize data when rendering with AGG or GD (except in a few special cases like when there are dashed lines).  By "generalize" I mean reducing the amount of data we actually render, based on the current zoom level.  For example, if I have an island with a border represented by 10000 vertices, and I'm zoomed out so the island only spans a few pixels, then we can remove some of these points and render the reduced set to get nearly the same visual effect.  In my tests I experimented with enabling generalization of data that was being rendered as polylines (polyline features, or the edges for polygon features).

4/ I did not do any performance testing.  For now I just wanted to focus on quality.  I expect that enabling point clamping by itself will have no noticeable impact on performance - it's just rounding values.  The performance impact of generalization would need to be investigated.


For those interested in this, please have a look at the images.  The zip file contains a number of folders, each with two images to compare.  Each folder also contains a text file describing what's being compared.

Please let me know what you think.  As far as options go: here's what I see:
*       do nothing: the change is not acceptable
*       enable just the point clamping changes
*       enabled both the point clamping changes and generalization (pending performance investigation)

Thanks,
Walt

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