Anti-aliasing bug (with text/labels)

[Steve Lime]

Curious bug... The value sent to the gdImageStringFT function is a color index not the color itself. 

For 8-bit images the first color allocated is always 0 and is the background color for the image. Typically you don't label with that color since labels would fade into the background, but you may outline labels with it.

For 24-bit images I don't know how GD arrives at the pen value. If there is a RGB value that is returned as 0 from the color allocation functions then there would be a problem.

I'm not sure how to address the bug since it's clearly a GD issue. They should work from an anti-aliasing flag rather than using the color index. The work around in either the 8-bit or 24-bit case would be avoiding the 0 color index if you really need to turn off anti-aliasing (e.g. avoid the IMAGECOLOR and black).

I'm open to suggestions...

Steve 

>>> Jani Averbach <javerbach at EXTENDTHEREACH.COM> 04/21/06 5:01 PM >>>
Hello, 

I have found a bug in Mapserver with anti-aliasing code when 
black text (#000000) is always anti-aliased.

The problem is that there is following kind of code in the 
mapgd.c::msDrawTextGD, around line 3090

...
gdImageStringFT(img, bbox,
((label->antialias)?(label->outlinecolor.pen):-(label->outlinecolor.pen)
),...
...

And the GD Manual says:

   The string is rendered in the color indicated by the gf color index. 
   Use the negative of the desired color index to disable anti-aliasing.

However, for pure black the pen is zero and there isn't such a thing as
negated zero... 
So black text is always anti-aliased regardless of if you are using
ANTIALIAS FALSE or not.
I have included a small map-file to demonstrate that bug.

BR, Jani


MAP
	IMAGETYPE PNG24
	SIZE 100 100 
	EXTENT 0 0 10 10
	UNITS METERS
	FONTSET   "../fonts/fonts.list"
	
	LAYER
		NAME NO_AA
		TYPE ANNOTATION
		STATUS DEFAULT
		FEATURE
			POINTS
				2 5
			END
			TEXT "a8j"
		END
		CLASS
			LABEL
				TYPE TRUETYPE
				FONT arial
				ANTIALIAS FALSE
				SIZE 20
				COLOR 0 0 0 
				#COLOR 1 1 1
			END
		END
	END
END

Jani Averbach
SRC,LLC

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