Dithered Edges when converting from PNG to GIF - RESOLVED

[mrshake]

I'm converting a PNG to GIF, and the dithering between the colored and transparent edges in the image are acting weird. It looks like the dither is choosing black as the trasition color... this is sticking out badly... can I control the color?

[anthony]

Can you give a link to an example image of the problem (before and after).

[fmw42]

I'm converting a PNG to GIF, and the dithering between the colored and transparent edges in the image are acting weird. It looks like the dither is choosing black as the trasition color... this is sticking out badly... can I control the color?

Note that GIF does not allow full 8-bit transparency. If your PNG has 8-bit transparency, then at least half will be lost to probably black. See

http://www.imagemagick.org/Usage/formats/#boolean_trans

[anthony]

GIF saving should not dither transparency by default in any case. It should be equivelent to a threshold of 50% of the alpha channel.
In any case 'black' should not be a result!

An example of the problem (images and command) will let us confirm just what is going on.

[mrshake]

Here is the image I'm starting with:



and the final gif with the bad edges

[anthony]

That is not dithering.

What it looks like is that you rotated the image on a black background then made anything perfectly black transparent. Naturally the edges will be blended with the black, resulting in what you see.

NOTE this is preferable if the GIF is to be displayed on a very dark background (such as on a web page). It will make the edge look smoother, and less jaged.

Suggestion. add -matte -background none before the rotate then when the image gets saved to GIF IM should do a automatic 50% transparency threshold (for boolean transparency).

That should get ride of the black.

For more help I need to see the command you are trying to use.

[mrshake]

Changing the matte doesn't seem to help... there is never a black background, nor one set... thats the tough part... this is all in PerlMagick... but here is the code:

Code: Select all

print "Set Density\n";
my $x15 = $agent4->Resample(density=>'300x300');
warn "$x4" if "$x4";
print "Set Units\n";
my $x16 = $agent4->Set(units=>'pixelsperinch', matte=>'True', mattecolor=>'none');
warn "$x4" if "$x4";
print "Set Rotation\n";
my $x17 = $agent4->Rotate(degrees=>'-5', background=>'none');
warn "$x4" if "$x4";
print "Set Scale\n";
my $x18 = $agent4->Scale(geometry=>'194x233');
warn "$x4" if "$x4";

print "Writing rotated temp file\n";
my $x19=$agent4->WriteImage($wftmp);
warn "$x19" if "$x19";

print "Composite rotated png onto transparent background\n";

my $im = Image::Magick->new();
my $bg = Image::Magick->new(size => "225x256");

my $rc = $bg->Read("xc:none");

$rc = $im->Read($wftmp);
die $rc if $rc;

#$rc = $bg->Set(density=>'300x300', matte=>'True', mattecolor=>'white');
$rc = $bg->Set(density=>'300x300');

$rc = $bg->Composite(gravity => "Center", image => $im);
warn $rc if $rc;

print "Write Temporary PNG file\n";

$rc = $bg->Write($wf2);
die $rc if $rc; 

print "Clearing Variables\n";

undef $agent1;
undef $agent2;
undef $agent3;
undef $agent4;
undef $agent5;
undef $im;
undef $bg;
undef $rc;


print "Convert PNG to GIF\n";

$im = Image::Magick->new();
$rc = $im->Read($wf2);

$rc = $im->Set(dither=>'False');

$rc = $im->Write($wf3);

[anthony]

ImageMagick version?

The default installation from some repositories are so old it pre-dates some very fundamental bug fixes.

Also the PNG image you supplied is the actiual source of the black color. The image contains semi-transparent pixels which has black mixed in with it. As it is semi-transparent, the color is then diluted with the background so you don't see the black.

That black color should NOT be present, and is probably a very VERY old 'black halo' type bug. I'll need the original unrotated image for testing.

You can see the black in the semi-transparent pixels by using...

Code: Select all

convert testagentws.png -channel A -threshold 99%  
testagentws_check.png

display -texture xc:white testagentws_check.png

No part of the above involves GIF, dithering, color quantization, or any of the things you though was the cause.

The black is in the semi-transparency of the source image. And semi-transparency should never contain any extraneous color such as black.

The more typical test for this is to rotate a pure white image, then when finished flatten it on white. If you see anything that is not white, we have a 'transparency halo' bug.


The current releast of IM does not have this bug for resize (see
Resize Halo Bug Fixed IM v6.2.4. Or one in rotate. here is the test for both.

Code: Select all

convert -size 100x100 xc:white -background none -rotate 5 -resize 50% -background white -flatten x:

result should be pure white.

[mrshake]

I'm currently running 6.4.6Q16 for IM

Here is the original image:

http://www.adamandnicole.com/gallery2/g ... f/test.tif

[anthony]

Okay I can validate that some operation in the latest IM still contains the 'halo' bug.

Your initial part of the perl script is equivelent to...

Code: Select all

convert test.tif -resample 300x300 -units pixelsperinch -matte -mattecolor none -background none -rotate -5 -scale 194x233 t.miff

Cutting this down and checking for semi-transparent black blending (the halo bug)...

Code: Select all

convert test.tif -resample 300x300 -matte  -background none -rotate -5 -scale 194x233 -channel A -threshold 99% -background white -flatten x:

Now -resample is done before the rotation so no transparency is involved. -rotate I know does not have a halo problem, so that leaves -scale.

Yeap -scale is at fault.

Code: Select all

convert -size 50x50 xc:white -matte -bordercolor none -border 20x20 -scale 5x5 -background white -flatten -unique-colors txt:-

# ImageMagick pixel enumeration: 3,1,65535,rgba
0,0: (54668,54668,54668,65535) #D58CD58CD58C rgba(83.418%,83.418%,83.418%,1)
1,0: (59062,59062,59062,65535) #E6B6E6B6E6B6 rgba(90.1228%,90.1228%,90.1228%,1)
2,0: (65535,65535,65535,65535) #FFFFFFFFFFFF white

This should produce a pure white image, but instead you get three colors (center, edge, corner) caused by the 'black halo' bug

As -resize halo effect was fixed, if you replace -scale with -resize, you get a pure white image and no black halo bug.

In Summery: -scale has the old 'halo bug' present.


As for you problem, replace image scale with proper image resize, something you should have been doing anyway!

[anthony]

Cristy just reported that he has now fixed the bug.

I have now also confirmed -scale, has been fixed with regard to the black halo bug.

[mrshake]

Brilliant!! The halo is gone!!

However, I'm still left with a problem... and its still the edges... here is what I'm ending up with, and then an example of what it needs to be:






The 2nd image is what my edges need to look like. I think the biggest issue here is that the original I'm trying to replicate was made in Photoshop with options that don't seem to translate well to IM. If I could convince them to move away from gif, it would make the whole thing easier.. but it needs transparency and is a web graphic... grrr

[anthony]

GIF does not allow semi-transparent edges. You either get jagged edges as you first image. OR you blend the semi-transparency with some color (like white or black) which was what you were getting before. Your original problem was with black your new example of what you 'want' is with white.

Solution. either rotate the image with a white background, or flatten the PNG image with a white background. After that you can then flood fill (either with -draw or the -floodfill operator) perfectly pure white to transparency. See IM examples, Color Modifications, Floodfill

For more examples for GIF handling see
GIFs on a solid color background

[mrshake]

it seems so simple.. yet it escapes me!!

here is what I added to the script just before the gif write as well as changing all the transparent backgrounds to white in the rest of the script.

$rc = $im->FloodfillPaint(geometry=>'0x0', fill=>'none');

the image when complete now appears to have a black background instead of transparent...

[anthony]

You must re-enable the transparent channel.. the color "none" is actually fully-transparent black, so if you don't have transparency, you just get black.

However DID you get the white blend in the edge? That is the question.