Legacy ImageMagick Discussions Archive

Use case: Social networks Twitter and Tumblr convert all PNGs to JPGs, sometimes substantially lowering their quality, unless the PNG has transparency. A single pixel being 'only' 99% opaque counts as transparency.

Is there any way to use ImageMagick to make a single pixel (e.g. 0,0) very slightly transparent without changing its RGB values? Just setting the A value to 0xFE for example. I can safely assume images have no transparency yet as I can detect images which do and upload as-is.

You have not said what your ImageMagick version is nor what platform you use. Syntax may differ. You also have not said if you need 24-bit PNG or 8-bit PNG

try Unix syntax to produce a 24-bit PNG file with the pixel 0,0 partially transparent

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convert logo: \
\( +clone -crop 1x1+0+0 +repage -alpha on -channel a -evaluate set 99% +channel \) \
-alpha on -channel rgba -geometry +0+0 -compose copy -composite logo.png

replace logo: with your image.png and logo.png with your image.png

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identify -verbose logo.png

Here are the last few entries of the histogram:

9: (255,242,193,255) #FFF2C1FF srgba(255,242,193,1)
31: (255,246,209,255) #FFF6D1FF srgba(255,246,209,1)
14: (255,249,224,255) #FFF9E0FF srgba(255,249,224,1)
256243: (255,255,255,255) #FFFFFFFF white
1: (255,255,255,252) #FFFFFFFC srgba(255,255,255,0.988235)

+++++++++++

Please, always provide your IM version and platform when asking questions, since syntax may differ. Also provide your exact command line and if possible your images.

See the top-most post in this forum "IMPORTANT: Please Read This FIRST Before Posting" at http://www.imagemagick.org/discourse-se ... f=1&t=9620

If using Imagemagick 7, then see http://imagemagick.org/script/porting.php#cli

For novices, see

http://www.imagemagick.org/discourse-se ... f=1&t=9620
http://www.imagemagick.org/script/comma ... essing.php
http://www.imagemagick.org/Usage/reference.html
http://www.imagemagick.org/Usage/
https://github.com/ImageMagick/usage-markdown

Sorry, Windows 10, IM7. That command does produce the behavior I'm looking for though, thanks!

Rewritten to Windows format:

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magick in.png ^
( +clone -crop 1x1+0+0 +repage -alpha on -channel a -evaluate set 99% +channel ) ^
-alpha on -channel rgba -geometry +0+0 -compose copy -composite out.png

AkemiHomura wrote: ↑2018-07-25T20:15:06-07:00A single pixel being 'only' 99% opaque counts as transparency. [...] I can safely assume images have no transparency yet as I can detect images which do and upload as-is.

If you know for sure the pixel at "0,0" is not already transparent, it can be as simple as this...

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magick in.png xc:gray95 -background none -compose copyopacity -composite out.png

That reads the input image, creates a 1x1 canvas 95% white, and composites it at 0,0 onto the input image. Using the compose method "copyopacity" should result in that top left pixel being 95% opaque, or 5% transparent, while maintaining its original color.

Note: If you composite that "xc:gray95" onto an area already transparent, it will result in an almost black pixel. You probably don't want that.

We can extend GeeMack's solution to cover cases where the input may already have transparency:

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magick in.png ( +clone -crop 1x1+0+0 -alpha extract -evaluate Multiply 0.99 ) -compose CopyOpacity -composite out.png

A simpler command that does the same thing is:

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magick in.png -alpha Set -region 1x1+0+0 -channel A -evaluate Multiply 0.99 +channel +region out.png

Legacy ImageMagick Discussions Archive

Changing a single pixel to be slightly transparent

Changing a single pixel to be slightly transparent

Re: Changing a single pixel to be slightly transparent

Re: Changing a single pixel to be slightly transparent

Re: Changing a single pixel to be slightly transparent

Re: Changing a single pixel to be slightly transparent