[mapguide-users] Render services image formats

[Trevor Wekel]

Hi Gary,

It is a well known fact that JPG introduces compression artifacts at hard image boundaries.  I suspect this will happen with text and labels.  PNG32 (just PNG in MapGuide) is a lossless compression format so there will be no artifacts introduced.

PNG8 is also lossless if have less that 256 colors in the image.  If you have tens of thousands of colors, there could be loss of detail.  I haven't tried this personally but rendering text and other vector data on top of detailed raster imagery could result in loss of detail.  When we convert to PNG8, we use a palette optimized for the image.  However, the detailed raster imagery will probably get more of the palette since it occupies more image pixels than the vector imagery.

If you are using raster base imagery, have you tried moving the base imagery into a tiled base layer?  The tiled base layer and "normal" layers are returned in separate map requests.  This would give you two independent color spaces and may improve the readability of text and labels.

Thanks,
Trevor

From: Gary Morin [mailto:gary.morin at keynetix.com]
Sent: Thursday, March 20, 2008 3:58 AM
To: mapguide-users at lists.osgeo.org
Subject: [mapguide-users] Render services image formats

Hi

I've been investigating the render services image formats in an attempt to improve performance over the internet

My first observation, it appears in MGOS 2 setting the image format in serverconfig.ini does not affect the formats being served, it was only when I edited the ajaxmappane.templ did I see a difference.

I know the default is PNG, but we can change it JPG and with MGOS2 PNG8.

The problem is, with PNG the map images look great, but the download size is enormous, I'm using some raster maps and the downloads are in the region of 1.5mb. on my internet connection it is taking in the region of 6 seconds to display the map after a pan.

If I switch to JPG the download is much faster, the downloaded images are around 250K, but the quality is poor, some text and labels are now unreadable.

I have also tried the new PNG8 format, but its results were very similar to JPG, image quality is poor.

Many of my clients are use to working with the old MG 6.5, even with raster files being served it performance was great.

Is there a way to alter the compression ratio of a format or any plans to allow this to be done in the future?

Thanks

Gary






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