Set transparent background using ImageMagick and c

Question

Suppose you have any image (PNG or JPG). This image has a white background and I need to make this background transparent.

I have tried with these examples:

• convert original.png -background none transparent.png
• convert original.png -background white -flatten -alpha off transparent.png

but with no desirable results.

How can I make it?

IMPORTANT: Using convert command-line.

Answer 1

Using ImageMagick, this is very similar to hackerb9 code and result, but is a little simpler command line. It does assume that the top left pixel is the background color. I just flood fill the background with transparency, then select the alpha channel and blur it and remove half of the blurred area using -level 50x100%. Then turn back on all the channels and flatten it against the brown color. The -blur 0x1 -level 50x100% acts to antialias the boundaries of the alpha channel transparency. You can adjust the fuzz value, blur amount and the -level 50% value to change the degree of antialiasing.

convert logo: -fuzz 25% -fill none -draw "matte 0,0 floodfill" -channel alpha -blur 0x1 -level 50x100% +channel -background saddlebrown -flatten result.jpg

Answer 2

You can Use this to make the background transparent

convert test.png -background rgba(0,0,0,0) test1.png

The above gives the prefect transparent background

Answer 3

Yep. Had this same problem too. Here's the command I ran and it worked perfectly: convert transparent-img1.png transparent-img2.png transparent-img3.png -channel Alpha favicon.ico

Answer 4

I am using ImageMagick 6.6.9-7 on Ubuntu 12.04.
What worked for me was the following:

convert test.png -transparent white transparent.png

That changed all the white in the test.png to transparent.

Answer 5

Solution

color=$( convert filename.png -format "%[pixel:p{0,0}]" info:- )
convert filename.png -alpha off -bordercolor $color -border 1 \
    \( +clone -fuzz 30% -fill none -floodfill +0+0 $color \
       -alpha extract -geometry 200% -blur 0x0.5 \
       -morphology erode square:1 -geometry 50% \) \
    -compose CopyOpacity -composite -shave 1 outputfilename.png

Explanation

This is rather a bit longer than the simple answers previously given, but it gives much better results: (1) The quality is superior due to antialiased alpha, and (2) only the background is removed as opposed to a single color. ("Background" is defined as approximately the same color as the top left pixel, using a floodfill from the picture edges.)

Additionally, the alpha channel 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 you can see I am blowing up the alpha channel to 200% before eroding.

Comparison of results

Here is a comparison of the simple approach ("-fuzz 2% -transparent white") versus my solution, when run on the ImageMagick logo. I've flattened both transparent images onto a saddle brown background to make the differences apparent (click for originals).

Notice how the Wizard's beard has disappeared in the simple approach. Compare the edges of the Wizard to see how antialiased alpha helps the figure blend smoothly into the background.

Of course, I completely admit there are times when you may wish to use the simpler solution. (For example: It's a heck of a lot easier to remember and if you're converting to GIF, you're limited to 1-bit alpha anyhow.)

mktrans shell script

Since it's unlikely you'll want to type this command repeatedly, I recommend wrapping it in a script. You can download a BASH shell script from github which performs my suggested solution. It can be run on multiple files in a directory and includes helpful comments in case you want to tweak things.

bg_removal script

By the way, ImageMagick actually comes with a script called "bg_removal" which uses floodfill in a similar manner as my solution. However, the results are not great because it still uses 1-bit alpha. Also, the bg_removal script runs slower and is a little bit trickier to use (it requires you to specify two different fuzz values). Here's an example of the output from bg_removal.

Answer 6

This works for me:

convert original.png -fuzz 10% -transparent white transparent.png

where the smaller the fuzz %, the closer to true white or conversely, the larger the %, the more variation from white is allowed to become transparent