[mapserver-users] differing image size with different mapserver versions

Rahkonen Jukka (MML) jukka.rahkonen at maanmittauslaitos.fi
Mon May 17 02:40:41 PDT 2021 

Hi,

I made a few tests with IrfanView and Gimp and it seems that the both images are antialized but the mapserv76 version is more fine-grained. The mapserv64 version has also less unique colors, 217 vs 250 colors. Saving the mapserv76 image with IrfanView or Gimp by using different settings does not reduce the file size much so the image obviously really has more details. Interlace mode (adam7) in Gimp made the image even bigger.

The visual quality of mapserv76.png is not so much better that it would justify the doubled file size. At least the area with dense rectangles on the right look better in mapserv64.png. But I fear that for any systematic further studies you should prepare a test case with source data and mapfile. There are very many small rectangles in the image and it may be that different antializing near the narrow black lines makes the difference. It would be interesting to test the output size with more typical maps.

-Jukka Rahkonen-

Lähettäjä: mapserver-users <mapserver-users-bounces at lists.osgeo.org> Puolesta Seth G
Lähetetty: maanantai 17. toukokuuta 2021 11.27
Vastaanottaja: Richard Greenwood <richard.greenwood at gmail.com>
Kopio: mapserver-users at lists.osgeo.org
Aihe: Re: [mapserver-users] differing image size with different mapserver versions

Hi Richard,

In your OUTPUTFORMAT declaration maybe try setting the DRIVER to AGG/PNG8
Also  IMAGEMODE PC256 was only for GD support that was removed in 7.0. Try IMAGEMODE RGB (or RGBA if you want transparency).

Seth

--
web:http://geographika.co.uk
twitter: @geographika


On Sat, May 15, 2021, at 8:00 PM, Richard Greenwood wrote:
I built from main and tested with ANTIALIAS ON and ANTIALIAS OFF. Assuming that my mapfile syntax below is correct, there is no difference in size and the resulting PNG is still approximately 3x larger than the MapServer 6.4 one.
  CLASS
    STYLE
      COLOR 255 255 208
      ANTIALIAS ON
    END
  END
Again, thank you for your suggestions.
Rich

On Sat, May 15, 2021 at 11:21 AM Even Rouault <even.rouault at spatialys.com<mailto:even.rouault at spatialys.com>> wrote:
Actually, I just tried to open mapserv64.png with gimp, and export it as a non-interlaced or interlaced PNG. And the non-interlaced version is 60 kB vs 96 kB for interlaced. So interlacing vs non-interlacing isn't the cause here.

Looking more closely at mapserv64.png  vs mapserv76.png , they are visually different. I suspect the rendering to be of higher quality in 7.6 due to anti-aliasing and perhaps better quantization to 8-bit. You could perhaps try to turn off antialiasing in your CLASS.STYLE. This has been re-vived very recently for AGG line rendering. It is master only however.
Le 15/05/2021 à 19:07, Richard Greenwood a écrit :
Even,

Thank you. I had ignored the difference in the interlacing of the two images because I didn't think it would have affected the size.

The maprgbapng.c file no longer exists in MapServer 6.4 or 7.6. There is an "if ( interlaced ..." code block in mapoutput.c that is the same in 6.4 and 7.6. I tried forcing it ON and rebuilding 7.6 but the output image still is not interlaced. Maybe that code is just a left over. Or maybe AGG doesn't support interlacing as https://www.mapserver.org/mapfile/outputformat.html seems to suggest.

In any case, thanks again.
Rich



On Sat, May 15, 2021 at 9:59 AM Even Rouault <even.rouault at spatialys.com<mailto:even.rouault at spatialys.com>> wrote:

Richard,

your 6.4 image is a interlaced PNG, whereas the 7.6 is a non-interlaced one.

I see in the git history a relevant commit:


$ git show 9984b39cc8f74d60eb240728df660a172a118aad
commit 9984b39cc8f74d60eb240728df660a172a118aad
Author: Thomas Bonfort <thomas.bonfort at gmail.com><mailto:thomas.bonfort at gmail.com>
Date:   Sun Oct 5 15:59:44 2008 +0000

    rgba_png: don't interlace by default


    git-svn-id: http://svn.osgeo.org/mapserver/trunk@7960 7532c77e-422f-0410-93f4-f0b67bdd69e2

diff --git a/maprgbapng.c b/maprgbapng.c
index 105d8d61..c3615edd 100644
--- a/maprgbapng.c
+++ b/maprgbapng.c
@@ -357,10 +357,13 @@ int msSaveImageRGBAQuantized(gdImagePtr img, gdIOCtx *ctx, outputFormatObj *form
     int bot_idx, top_idx;
     int remap[256];
     int reqcolors = atoi(msGetOutputFormatOption( format, "QUANTIZE_COLORS", "256"));
+    const char *interlace;
     ms_png_info info;
     info.width = gdImageSX(img);
     info.height = gdImageSY(img);
-    if( strcasecmp("ON", msGetOutputFormatOption( format, "INTERLACE", "ON" )) == 0 )
+    interlace = msGetOutputFormatOption( format, "INTERLACE", "OFF" );
+    if( strcasecmp("ON", interlace) == 0 || strcasecmp("YES", interlace) == 0
+            || strcasecmp("1", interlace) == 0)
         info.interlaced=1;
     else
         info.interlaced=0;



But this predates 6.4 release by several years, so this doesn't explain why you see a different behavior, unless something in 6.4 still turned on interlacing on in that configuration.

From what I can see in the doc, the interlacing mode was removed in the 7.0 release when GD went off, so I don't think you can do much. I guess that could be re-added but would require some coding.

Even


Le 15/05/2021 à 17:43, Richard Greenwood a écrit :
I get significantly different image sizes between MapServer 6.4 and 7.6 with the same output format definition. I've tried many variations of the following.

OUTPUTFORMAT
  NAME "png-test"
  DRIVER "AGG/PNG"  # GD driver is same (6.4 only)
  MIMETYPE "image/png; mode=8bit"
  IMAGEMODE PC256
  FORMATOPTION "QUANTIZE_FORCE=on"
  FORMATOPTION "QUANTIZE_COLORS=256"
  EXTENSION "png"
  # TRANSPARENT on or off makes little difference
END

MapServer 6.4<https://greenwoodmap.com/mapserv64.png> is 90kB, MapServer 7.6<https://greenwoodmap.com/mapserv76.png> is 320kB. Any suggestions as to how I can get my MapServer 7.6 image sizes down closer to what I'm getting in MapServer 6.4?

Thanks,
Rich

--

Richard W. Greenwood, PLS
www.greenwoodmap.com<http://www.greenwoodmap.com>


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--

Richard W. Greenwood, PLS
www.greenwoodmap.com<http://www.greenwoodmap.com>

--

http://www.spatialys.com

My software is free, but my time generally not.


--
Richard W. Greenwood, PLS
www.greenwoodmap.com<http://www.greenwoodmap.com>
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