possible bug in -border with transparency IM 6.5.2-4

[fmw42]

IM 6.5.2-4 Q16 Mac OSX Tiger.

I was testing my texteffect script with transparent background for a user and ran into this puzzling side-effect. I am basically doing a blur text effect with text color opaque blue rgba(0,0,255,1) and transparent white background rgba(255,255,255,0). The problem is that at the very end, just before the -border step (where I try to add back a transparent white border) all is fine, but afterwards much of the image turns black (like an undercolor of black) after transparency is removed. I cannot figure out if I am making a mistake, do not understand something critical or if there is a bug in -border when used with transparency. I cannot duplicate this by adding, trimming and adding back a transparent white background around the rose image. Thus I am providing the following, which is as simple as I can make it. See the images produced at various stages before and after turning off transparency.


convert -size 139x57 xc:"rgba(255,255,255,0)" \
-fill "rgba(0,0,255,1)" -stroke none \
-font Arial -pointsize 48 \
-gravity center -annotate 0x0+0+0 "TEST" -blur 0x3 -write texteffect_tmp1.png \
-trim +repage -write texteffect_tmp2.png \
-bordercolor "rgba(255,255,255,0)" -border 5 texteffect_tmp3.png


convert texteffect_tmp1.png -alpha off texteffect_tmp1_aoff.png




convert texteffect_tmp2.png -alpha off texteffect_tmp2_aoff.png




convert texteffect_tmp3.png -alpha off texteffect_tmp3_aoff.png


I hope someone can explain this to me where the black is coming from (and if possible how to avoid that without leaving off the -trim ... -bordercolor ... -border). Thanks

Fred

[anthony]

I hope someone can explain this to me where the black is coming from (and if possible how to avoid that without leaving off the -trim ... -bordercolor ... -border). Thanks

Any pixel with is transparent has a completely undefined color!

However PNG files will still save that color even though it is fully-transparent.

As such texteffect_tmp2.png contains the text color blurred with white in the transparent areas
But texteffect_tmp3.png has the fully-transparent pixel color set to black (very typical handling for fully-transparent pixels).

The black in this case was caused by the alpha composition method "over" that the -border operator used to add an extra border to the image. It is Correct handling!

On technique that could fix this is to 'turn off the transparency befor using -border (-alpha off) but then just turn it on again (-alpha off) afterward. As alpha is turned off while border is being applied, then it should (I hope) be ignored and preserved, and remove its effect on the composition.

Code: Select all

convert texteffect_tmp2.png -alpha off -bordercolor white -border 5 -alpha on texteffect_tmp3.png
convert texteffect_tmp3.png -alpha off texteffect_tmp3_aoff.png

YEAP that works perfectly, the colors were preserved as they were thought of as being opaque (alpha channel was switched off) for the duration of the -border and its internal alpha composition.

Tricky but does the job.

WARNING: a PNG with variable color for hidden by full transparency does not compress as well as one where that color was reset to black (or some other color).

See Discussion in
viewtopic.php?t=13746
and the addition to IM examples in
http://www.imagemagick.org/Usage/basics/#alpha_color

[fmw42]

Thanks.

Fred