Antialiasing to JPEG

[warmerdam]

Thanx Flavio to reopen a subject that I investigated more than 1 year ago and
pushed me to re-check the situation with newer relases of Mapserver and GDAL.
Though the Jpeg outputformat is much lighter than the Png24 I experienced a
problem with  truetype point symbols. I don't know why but with the jpeg
outputformat they are shown in a very poor quality. Instead using the same
mapfile, just changing the outputformat to Png24, the truetype symbols are
perfect. The Jpeg quality doesn't influence this behaviour, even if i set it to
100 it's the same.
So if I want perfect symbols I have to set the outputformat to png24 but in this
way the file is much bigger. Otherwise I have to renounce the truetype symbols
and use only the other types of point symbols. Is this a limitation of the Gdal
libraries?
For what regards the line antialiasing I noticed, though not measuring it, that
the performance is strongly affected by the use of a cartoline line symbol even
if the outputformat is set to Jpeg.
The antialiasing is an interesting feature and the desktop systems (ex.
Manifold) that uses a good antialiasing engine really have a very attractive
look but, depending of the project goals, this feature could be considered of
secondary importance compared to performance, especially in case or many remote
accesses and not so fast connections such as in my country.
 
Giorgio Plazzotta


...

Hi Frank

attached the data to test (change the PATH for data and symbols in the
mapfiles).

you can see the effects on:
http://www.mapfinder.ch/start.php?site=temp
(click on the 1024x768 to launch)

Flavio

[fwarmerdam]

mapfile and data from Flavio

[fwarmerdam]

OK, so I have determined this affects any RGB mode output including PNG24.
It does not affect PC256 or RGBA mode output. 

The problem is basically that any pixel touched by the symbol is set to the 
symbol color 100% in RGB mode.  With alpha blending enabled, alpha blending
occurs and things look OK.  This can be forced by adding the 
  TRANSPARENCY ALPHA 

line in the LAYER definition.

I have also published an FAQ on this issue on the new web site. 

However, I am still not 100% satisfied, and think that perhaps the
actual symbol rendering code ought to be doing alpha blending by
default just for symbol rendering, instead of using the TRANSPARENCY
keyword to force rendering into a temporary buffer ... a fairly heavy
weight solution.

I'll leave this bug open for now.