gm convert -size 200x200 xc:"transparent" -fill "#4D3681" -stroke "#FF0000" -draw "ellipse 100,100 50,100 0,360" test1.png
draws an opaque unwanted black ring about the ellipse
gm convert -size 200x200 xc:"transparent" -fill "rgba(255,255,255,255)" -stroke "#FF0000" -draw "ellipse 100,100 50,100 0,360" test2.png
draws an opaque unwanted white ring about the ellipse
How to get an ellipse painted on a transparent background with an antialiased translucent border?
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
And, not too surprisingly as I am sure they use the same internal mechanisms, an svg ellipse has the same artifacts.
I am thinking of two workarounds:
1. Make a custom shapes font and skew it with svg. It seems that fonts do not have the pixel halo that shapes do. This would be an ardous task trying to get positions correct, adjusting for baselines and font sizes and ugh.
2. Render the shape to black on white, then the color I want my shape on white, and use those two to punch out a composite.
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
So... before I go working around this... is this a known bug or am I doing something wrong ?
How can I help GM?
drawing ellipse on transparent canvas creates unwanted 'halo'
I am sure that this is a known issue. The problem is that the
blending is always dealing with two colors (canvas image pixel color
and pen/drawing color). Even if the image appears to be transparent,
it still has color (e.g. black), even if it can't be seen after alpha
composition on top of another image. It is not really possible to
anti-alias or blend without considering the color of what is being
drawn on. If the underlying color is transparent, then it should not
contribute to the computed result. I think that this is what the
existing math is trying to do.
If the image is 50% transparent and black, then the result is going to
be a modulation between the pen color and black, but reducing the
influence of the black by 50%. But the result still contains black
which would be unwanted if composited on top of an image of a
different color.
The drawing code uses AlphaCompositePixel() in
magick/alpha_composite.h. In order to help GM, someone would need to
study AlphaCompositePixel() and determine improved math which solves
the fringing problem while not breaking other usages. If it does
break other usages, then a new function is needed to replace
AlphaCompositePixel() in the render code.
I have seen some images where drawing was partially done in the
opacity channel rather than the color channels. This could be a
solution but makes the resulting image not very usable for other
purposes.
I'm just running into this myself. I'm attempting to create intermediary images that contain some "annotations". Then, I was planning to create a composite image of all the "annotation images" on top of the target image. Only, the annotation images all end up with that halo effect on the edges.
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
I'm just running into this myself. I'm attempting to create
intermediary images that contain some "annotations". Then, I was
planning to create a composite image of all the "annotation images"
on top of the target image. Only, the annotation images all end up
with that halo effect on the edges.
This problem has not been studied or worked on yet. Similar issues
have been solved in other areas such as when resizing an image, which
also produced fringing.
I am not sure if the SVG specification addresses drawing on
transparent backgrounds.
gm convert -size 200x200 xc:"transparent" -fill "#4D3681" -stroke "#FF0000" -draw "ellipse 100,100 50,100 0,360" test1.pngdraws an opaque unwanted black ring about the ellipse
gm convert -size 200x200 xc:"transparent" -fill "rgba(255,255,255,255)" -stroke "#FF0000" -draw "ellipse 100,100 50,100 0,360" test2.pngdraws an opaque unwanted white ring about the ellipse
How to get an ellipse painted on a transparent background with an antialiased translucent border?
And, not too surprisingly as I am sure they use the same internal mechanisms, an svg ellipse has the same artifacts.
I am thinking of two workarounds:
1. Make a custom shapes font and skew it with svg. It seems that fonts do not have the pixel halo that shapes do. This would be an ardous task trying to get positions correct, adjusting for baselines and font sizes and ugh.
2. Render the shape to black on white, then the color I want my shape on white, and use those two to punch out a composite.
So... before I go working around this... is this a known bug or am I doing something wrong ?
How can I help GM?
On Mon, 23 Nov 2015, Erik wrote:
I am sure that this is a known issue. The problem is that the
blending is always dealing with two colors (canvas image pixel color
and pen/drawing color). Even if the image appears to be transparent,
it still has color (e.g. black), even if it can't be seen after alpha
composition on top of another image. It is not really possible to
anti-alias or blend without considering the color of what is being
drawn on. If the underlying color is transparent, then it should not
contribute to the computed result. I think that this is what the
existing math is trying to do.
If the image is 50% transparent and black, then the result is going to
be a modulation between the pen color and black, but reducing the
influence of the black by 50%. But the result still contains black
which would be unwanted if composited on top of an image of a
different color.
The drawing code uses AlphaCompositePixel() in
magick/alpha_composite.h. In order to help GM, someone would need to
study AlphaCompositePixel() and determine improved math which solves
the fringing problem while not breaking other usages. If it does
break other usages, then a new function is needed to replace
AlphaCompositePixel() in the render code.
I have seen some images where drawing was partially done in the
opacity channel rather than the color channels. This could be a
solution but makes the resulting image not very usable for other
purposes.
Bob
Bob Friesenhahn
bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
I'm just running into this myself. I'm attempting to create intermediary images that contain some "annotations". Then, I was planning to create a composite image of all the "annotation images" on top of the target image. Only, the annotation images all end up with that halo effect on the edges.
On Fri, 6 Mar 2020, Ben Nadel wrote:
This problem has not been studied or worked on yet. Similar issues
have been solved in other areas such as when resizing an image, which
also produced fringing.
I am not sure if the SVG specification addresses drawing on
transparent backgrounds.
Bob
Bob Friesenhahn
bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
Public Key, http://www.simplesystems.org/users/bfriesen/public-key.txt
Sounds good :thumbs-up: