Imagemagick: remove alpha component (replace all intermediate alpha pixel with solid pixel)

Question

To solve Android build issue I need to replace all intermediate alpha pixel with solid pixel (leaving transparent background as is).

How to that with ImageMagick or other-command line tool to all images in a tree?

Image bg_all_block.9.png

Image btn_bg_common_press.9.png

UPDATE: I have found that I can detect if alpha is used, as in Detect Alpha Channel with ImageMagick

Other found links

• https://graphicdesign.stackexchange.com/questions/16120/batch-replacing-color-with-transparency
• http://www.imagemagick.org/Usage/color_basics/#replace

Negligent

Answer 1

To remove the alpha channel from single image use this command:

convert input.png -alpha off output.png

To remove the alpha channel from all images inside a folder, make use find to first find all PNG files, and then run 'm through convert:

find . -name "*.png" -exec convert "{}" -alpha off "{}" \;

Please test on a COPY of your files to be sure.

...

see dialog below, and the answer is based on that "we need to remove alpha that is not 255"

convert input.png -channel A -threshold 254 output.png

and for batch

mkdir batch
FOR %G IN (*.png) DO convert %G -channel A -threshold 254 batch\%G

Liturgy

I don't do much Windows but it's probably mogrify -alpha off *.png. Not sure if mogrify has a -r switch for recursing through directories or if it will take */*.png – Liturgy

Well, I found that this also removes transparent background for my icons. While I need only process those pixels with intermediate alpha... imagemagick.org/Usage/color_basics/#replace hints at some convert balloon.gif -fill white -opaque blue balloon_white.gif Now looking at what -opaque means – Negligent

Try -channel A -threshold 254 – Liturgy

Can you post an image? – Liturgy

That is clever to say "we need remove alpha that is not 255". I tried with convert bg_all_block.9.png -channel A -threshold 254 -alpha off bg_all_block_254.9.png, but it changes background – Negligent

Yes, this is btn_bg_common_press.9.png – Negligent

Sorry, I meant for the -channel A -threshold 254 to fully replace the -alpha off, so like this convert input.png -channel A -threshold 254 output.png – Liturgy

You might get on better experimenting with different percentages, like this convert input.png -channel A -threshold 30% output.png – Liturgy

Thank you so much. I have put final result into updated answer and accepted it. ImageMagick is so huge. – Negligent

My pleasure, glad to have been of assistance - sorry I was a bit slow to get to the right solution today. – Liturgy

For macOS users - first you have to install imagemagick. You can do this using brew: brew install imagemagick – Shrewish

When I run convert input.png -alpha off output.png, I get an image that is all black. How do I convert transparency to white? I tried convert input.png -background white -alpha off output.png with no visible effect. – Concentric

But this one works: convert input.png -background white -alpha remove -flatten -alpha off output.png https://mcmap.net/q/86264/-replace-transparency-in-png-image-with-white-background – Concentric

convert: no images defined ./0.png' @ error/convert.c/ConvertImageCommand/3300. convert: no decode delegate for this image format PNG' @ error/constitute.c/ReadImage/556. – Tuberosity

Answer 2

What worked for me on macOS for batch processing was:

for f in *.png; do convert "$f" -channel A -threshold 254 "${f%%.png}.png"; done

Kenner

Answer 3

To remove alpha channel from all pictures in the folder (f.ex. all .png files) I use following command (in terminal on macOS):

for file in *.png; do convert $file -alpha deactivate; done

Unfortunately, none of any other solution given in this thread worked for me.

Binucleate

Answer 4

To replace alpha channel with solid white color in PNG with Image Magick, the following works for me:

$ convert -flatten f.png f.white-background.png

Tocantins