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Resizing PNG loses transparency
Posted: 2017-03-09T23:46:29-07:00
by MrChilBoy
I'm not sure what I'm doing wrong here but when I resize using the following command I lose the transparency of my PNG file.
This works fine (step one)
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magick convert logosrc.png -trim sample.png
...but this creates a white background (step 2).
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magick convert logosrc.png -trim -resize 600x600 sample.png
When I look at the identify -verbose of the original file and the file produced by step one, the following identifies the format (I think).
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Type: PaletteAlpha
Base type: TrueColorAlpha
It changes to the following after step 2 (and co-incidentally for another image I'm having issues with)
Using ImageMagick 7.0.5-1 Q16 x64 on Windows 10
Please help
Re: Resizing PNG loses transparency
Posted: 2017-03-10T00:46:59-07:00
by fmw42
After -trim add +repage. Does that help? If not, upload an example image to some free hosting service such as dropbox.com and put the URL here.
Also you are running the IM 6 legacy code by putting in convert. Remove the convert and just use magick.
Re: Resizing PNG loses transparency
Posted: 2017-03-10T02:05:21-07:00
by MrChilBoy
Ok, turns out to be a weird bug in Picasa Photo Viewer ...sort of.
The results :
Original file ->
Resulting file ->
They look the same to some viewers but the identify -verbose outputs are different.
If I use the following code though, the identify -verbose ends up almost identical.
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magick logosrc.png -strip sample.png
Anyway, thanks for the tip about just using magick
Re: Resizing PNG loses transparency
Posted: 2017-03-10T02:14:59-07:00
by MrChilBoy
Re: Resizing PNG loses transparency
Posted: 2017-03-10T11:02:56-07:00
by fmw42
The two images are identical in pixel data.
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compare -metric rmse logosrc.png sample.png null:
0 (0)
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identify logosrc.png sample.png
logosrc.png PNG 197x201 197x201+0+0 8-bit sRGB 16.3KB 0.000u 0:00.000
sample.png PNG 197x201 197x201+0+0 8-bit sRGB 14.7KB 0.000u 0:00.000
The IM converted file may be slightly smaller size, because its zip compression may more efficient than the result from whatever tool you used
The IM converted file has three extra PNG meta tags:
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png:bKGD: chunk was found (see Background color, above)
png:cHRM: chunk was found (see Chromaticity, above)
png:gAMA: gamma=0.45454544 (See Gamma, above)
So perhaps a different version of libpng was used. What tool created your original PNG?
Try adding an sRGB profile to each and see if they view the same. When I do:
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convert logosrc.png -profile /Users/fred/images/profiles/sRGB.icc logosrc_prof.png
convert sample.png -profile /Users/fred/images/profiles/sRGB.icc sample_prof.png
then they view the same.
