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https://github.com/hackerb9/mktrans

Make background transparent using antialiased alpha channel
https://github.com/hackerb9/mktrans

Last synced: about 1 year ago
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Make background transparent using antialiased alpha channel

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README 

          
Astrolabe



# mktrans



Convert images into shaped transparent pngs by floodfilling the

background with transparency (antialiased alpha channel).



Top left pixel is assumed to be the background color and floodfill

is from image edges, unless a different starting pixel is specified.



## Example



This program is meant to be quick and simple to use, so most of the

time simply specifying a filename, such as foo.jpg, is all you need.



    mktrans foo.jpg



(Result will be in foo*-transparent.png*.)



If too much or too little of the image is turning transparent, you can

specify a different "fuzz" factor.



    mktrans -f 10 foo.jpg



## Sample results



For a quick sample using the ImageMagick logo, run these commands:



    convert logo: logo.png

    mktrans logo.png

    display logo-transparent.png



[![After flattening on a saddle brown background](https://i.imgur.com/Exrm0tD.png)](https://i.imgur.com/PReCAca.png)



## Installation



It's just a shell script, so just download the mktrans file, mark it executable, and put it in your bin directory.



    wget https://github.com/hackerb9/mktrans/raw/master/mktrans

    chmod +x mktrans

    mv mktrans /usr/local/bin/



Note: You will need to have [**imagemagick**](https://imagemagick.org/index.php) for it to work. You can find instructions on how to download it [here](https://imagemagick.org/script/download.php).



## All flags and complete usage



    Usage: mktrans [-f ] [-s|-S] [-v] 



        -f : How loosely to match the background color (default 20%)

               -s: Use speedy antialiasing (much faster, slightly less acurate) 

               -S: Do not antialias transparency (useful for repeated runs)

       -p ,: Floodfills from pixel at x,y instead of 0,0

               -v: Verbose



Output filenames will be the same as input, except suffixed with

`-transparent.png`. E.g., `mktrans foo.gif bar.jpg` creates

`foo-transparent.png` and `bar-transparent.png`.



### About -f *fuzz*



*Fuzz* is how far off the background color can be (in percent). You

usually won't have to change this. If fuzz is too high, parts of the

foreground image will be missing. If fuzz is too low, parts of the

background will not be removed. On certain images it may help to tweak

the fuzz level to get good antialiasing. (Not losing too much of the

edge, nor leaving any halos).



### About -s (speedily skip subpixels)



By default this script creates an antialiased (blurred) alpha channel

that is also eroded by half a pixel to avoid halos. Of course,

ImageMagick's morphological operations don't (yet?) work at the

subpixel level, so I'm blowing up the alpha channel to 200% before

eroding. Since this can be slow on large images, consider using the

'-s' option which tries to antialias without the subpixel eroding.



### About -S (Supress antialiasing completely)



Similar to -s, but does not antialias at all. Useful with with -p, below.

See an example of why you'd use -S in [astrolabe.md](astrolabe.md).



### About -p *x*,*y*



*X*,*y* specifies which pixel to start floodfilling from, instead of 0,0.

You probably should just _ignore_ this option and use The GIMP for

more fiddly, complex images.



Theoretically you can use -p to remove any lagoons that got missed by

a first floodfill. Note the letters 'a' and 'g' in the example below.



    convert logo: foo.png

    mktrans -p 160,100 foo.png

    mv foo-transparent.png foo.png

    mktrans -S -p 170,80 foo.png

    mv foo-transparent.png foo.png

    mktrans -S -p 195,65 foo.png

    mv foo-transparent.png foo.png

    mktrans -S -p 260,60 foo.png

    mv foo-transparent.png foo.png

    mktrans -S -p 285,55 foo.png

    mv foo-transparent.png foo.png

    mktrans foo.png

    mv foo-transparent.png logo-transparent.png

    

[![Using -p to fill lagoons](https://i.imgur.com/Hxl1a1A.png)](https://i.imgur.com/CmbUnHk.png)



You can find the correct coordinates for a pixel by using

ImageMagick's `display` command and then middle-clicking on the image.

Unfortunately, ImageMagick has no way to use that feature from a shell

script. If it did, this script could simply ask you to click on all

the start points of the image.



## Bugs



* The -p option is ugly and probably nobody wants to use it. Maybe I

  should remove it just to make the documentation shorter and clearer.

  Also, nearly every single bug listed here is due to -p existing.



* Running this script on an image that already has transparency will

  erode the image due to the antialiasing. Using -S is a workaround,

  but is not very satisfactory. Perhaps this script should remove any

  existing transparency before manipulating the image and then add it

  back in at the end. But then again, how often are people going to

  want to do that? The only use I can think of is when using -p.



* Because of the previous bug, if you do use -p to fill lots of

  lagoons, you'll probably want to use -S at the same time.

  (Should this be done automatically if the image already has

  transparency? Yuck, more complexity.)



  See the [astrolabe](astrolabe.md) example where running mktrans a

  second time on a specific pixel requires the use of -S so as to

  not cause the image to be eroded.

    

            mktrans a.png

            mktrans -p 258,54 a-transparent.png

            mv a-transparent-transparent.png a-transparent-transparent' without -S).png

            mktrans -S -p 258,54 a-transparent.png

            mv a-transparent-transparent.png a-transparent-transparent' with -S).png



* Finding the coordinates for -p is a pain. (Tip: middle click on

  image when viewing with `display`). It'd be nice if there was a -P

  option which let the user click on a point (or multiple points) in

  the image to start the floodfill.



* Instead of choosing coordinates, we could use ImageMagick's

  connected components labeling (blob extraction) to give people a

  choice of which lagoons to fill. 



## See Also



This is similar to ImageMagick's

["bg_removal"](https://www.imagemagick.org/Usage/scripts/bg_removal)

script, but much higher quality. (It's also faster and simpler to use.)