How to keep transparency with "-compose" and "-flatten" on GraphicsMagick

Question

I have a series of PNG images with transparent backgrounds that I want to overlay onto a destination PNG image that also has a transparent background. For argument sake let's say that the series of images are : - Source images : img1.png, img2.png, img3.png - Destination image : dest.png. Since I want to overlay all the images at once, I am going to use the `convert` command, with the `compose` switch as follows : ``` gm convert -compose Atop dest.png img1.png img2.png img3.png -flatten output.png ``` Seems simple enough, but the problem is that output.png looses it's transparency and I do not know how to keep it enabled. If I use the `-background` switch I can set what used to be the transparency to any color I want, but I cannot get it to go back to transparency. Yes, I can subsequently call: ``` gm convert -transparency black ouput.png output2.png ``` but then any black on the actual image becomes transparent as well. Any help here?

Accepted Answer

I faced the same issue, and I did a couple things to get it to work.

You should get what you want if you change your command to:

```
gm convert xc:transparent -compose Over img1.png img2.png img3.png -mosaic dest.png
```

You may want to use `Atop` of `Over` depending on the functionality you want out of the compose method.

Using `mosaic` instead of `flatten` combined with the `xc:transparent` argument caused it to keep their transparencies and produces an image with transparencies.

How to keep transparency with "-compose" and "-flatten" on GraphicsMagick

Solution

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