Replace transparency in PNG images with white back

Question

I've got some PNG images with transparency, and I need to create versions with the image layer composed onto a white background. I've tried various things with Image Magick "convert" operations, but either nothing happens at all or I get an error. I don't want to go to an intermediate JPG form because I don't want the artifacts. Of course it's easy to do this in Gimp or Photoshop or whatever, but I'd really rather script it from the command line because there are many of these things.

An example of a non-working Image Magick command is:

convert img1.png -background white -flatten img1-white.png

That results in an error.

Thanks!

Answer 1

Flattening image and applying background image is straight forward in ImageMagick

However, order of the commands is very important

To apply any background on a transparent image and flatten it, first apply the background than flatten it. The reverse doesn't work.

$ convert sourceimage.png -background BackgroundColor -flatten destinationimage.png

Answer 2

I saw this question and answers which really help me but then I was needed to do it for a lot of files, So in case you have multiple images (PNG images) in one folder and you want to do it for all:

find ./ -name "*.png" -exec convert {} -flatten {} \;

Answer 3

The Alpha Remove section of the ImageMagick Usage Guide suggests using the -alpha remove option, e.g.:

convert in.png  -background white  -alpha remove  out.png

...using the -background color of your choosing.

The guide states:

This operation is simple and fast, and does the job without needing any extra memory use, or other side effects that may be associated with alternative transparency removal techniques. It is thus the prefered way of removing image transparency.

It additionally adds the note:

Note that while transparency is 'removed' the alpha channel will remain turned on, but will now be fully-opaque. If you no longer need the alpha channel you can then use Alpha Off to disable it.

Thus, if you do not need the alpha channel you can make your output image size smaller by adding the -alpha off option, e.g:

convert in.png  -background white  -alpha remove  -alpha off  out.png

---

There are more details on other, often-used techniques for removing transparency described in the Removing Transparency from Images section.

Included in that section is mention of an important caveat to the usage of -flatten as a technique for removing transparency:

However this will not work with "mogrify" or with a sequence of multiple images, basically because the "-flatten" operator is really designed to merge multiple images into a single image.

So, if you are converting several images at once, e.g. generating thumbnails from a PDF file, -flatten will not do what you want (it will flatten all images for all pages into one image). On the other hand, using the -alpha remove technique will still produce multiple images, each one having transparency removed.