[SOLVED] convert to gradual alpha channel

[Art60]

Hi,

I'm a new IM user and I can't see to find how to convert a black background into a transparent one AND compensate antialiasing (black blending)

I found in this forum that the command line I needed was something like
convert original.png -transparent black x_copy.png

But the original image had antialiasing so the thin shape sometimes blends into the black background.


So the resulting picture has dark edges around the shape.


I think this alpha creating process could be enhanced by, as an option, beeing gradual and more, correct pixel color in order to compensate the color it erases.

Or maybe this process already exists and I didn't find how to activate it.

Thank you!
Art.

[fmw42]

You need to create a binary mask, then blur it a litte, transform the blur so that 100 stays 100, 50 and below goes to 0 (black) so that the partial transparency does not bleed into the black areas, then composite the original with the mask into the alpha channel.

try


convert original.png \( -clone 0 -fill white +opaque black -blur 0x0.5 -level 50,100% \) \
-alpha off -compose copy_opacity -composite original_trans2.png

You can play with adjusting the 0.5 blur sigma to fine tune the transparency
The -fill white +opaque makes everything not black into white

see

http://www.imagemagick.org/Usage/basics/#image_seq
http://www.imagemagick.org/Usage/compose/
http://www.imagemagick.org/Usage/compose/#copyopacity
http://www.imagemagick.org/Usage/layers/#convert
http://www.imagemagick.org/Usage/color/#level

[anthony]

Actually to recover the alpha correctly you need to determine amout of 'alpha' each pixel has,
and also color correct (remove alpha amount of black) from the color of each pixel.

This is a very tricky problem which is very difficult to do.

[Art60]

In fact, Anthony, this is the deal :
I know background is black
I know wave form should be "pure" color
So, I should determine the alpha value: easy, this is directly the pixel darkness, at the other hand, the color is directly the corresponding saturated one.

Thx!

Fmw42, your answer seems to be very complex at first glance and for what I understood plays with blur. Doesn't seem this cartesian to me

For your information, my "working" conversion batch is:

Code: Select all

convert "%~s1" -transparent black "%~dpn1_x.png"

So, I tried to write down, without trying to understand, your command line in a similar conversion batch file:

Code: Select all

convert "%~s1" \( -clone 0 -fill white +opaque black -blur 0x0.5 -level 50,100% \) \ -alpha off -compose copy_opacity -composite "%~dpn1_x.png"

But it throws errors:
Magick: unable to open image `\(': No such file or directory @ error/blob.c/OpenBlob/2572.
Magick: no decode delegate for this image format `\(' @ error/constitute.c/ReadImage/532.
Magick: option requires an argument `-level' @ error/convert.c/ConvertImageCommand/1773.

So I removed backslashes:

Code: Select all

convert "%~s1" ( -clone 0 -fill white +opaque black -blur 0x0.5 -level 50,100% ) -alpha off -compose copy_opacity -composite "%~dpn1_x.png"

But I still get an error:
Magick: option requires an argument `-level' @ error/convert.c/ConvertImageCommand/1773.
I don't understand much for -level is included :/

This is too complex for me as a first approach, sorry.
I think I'll undestand this when I have a week or two of writing scripts...

Thk you very much for your efforts, though, much appreciated.

Best regards,
Art.

[anthony]

The use of '\' to 'escape' the parenthesis is needed for UNIX shells, and PHP shell escapes. IT is not needed for windows.

For Unix shell to window differences see IM Examples, Windows Usage, Conversion
http://www.imagemagick.org/Usage/windows/#conversion


As for determining your alpha values. As you are using saturated colors on a black background you may be able to use the results of of of the Hue colorspaces, such as HSL. The 'S' channel (green channel normally) is the color saturation and make give a good representation of just how transparent each pixel should be. The 'L' or luminance channel may also be usable. The 'H' channel would then tell you what the actual color of the transparent pixel should be.

Note that these are just guesses on my part, but would be a good starting point for the specific image you have given as an example.

[Art60]

HSL !
This is THE trick, Anthony

Yes, I think this is the way I have to compute things with this kind of images.
A<-L
L<-maxvalue
S<-maxvalue
or something like that, may be
A<-maxvalue-L
should work better, I don't know which sign is Alpha vs transparency/opacity

You're guiding me to the clarity, dear friend

[fmw42]

Seems to me that your colors are anti-aliased against black, so some bleeding of the colors into the black will occur which you cannot remove perfectly and won't know an exact mixture as it depends upon the anti-aliasing method. So what I tried to do was blur the mask used for transparency just a little to make the edges of your colors slightly transparent where black mixed in with the color and to remove the color bleed into the black areas.

Can you not make your line drawing directly onto a transparent background rather than a black one?

[Art60]

fmw42,

You may be true, I didn't succeed in running your script...
If you can help me in running it, may be I could tweak it a bit.
Thx!

I'm not the author of the pics.
These are directly from http://www.freesound.org/index.php pages

I won't ask them to draw all their samples with alpha instead of black background
In fact I did , but I'm pretty sure they don't need to, so I have to do with what exists.

Thank you very much for your attention.
Art.

[fmw42]

I don't understand your usage of %~ in the following:

convert "%~s1" -transparent black "%~dpn1_x.png"

I suspect that is Windows syntax and I use a Mac and don't really know Windows scripting.

If you are on unix or mac then just take one image (not some wildcard series of images as IM does not do that well with convert) and run

convert original.png \( -clone 0 -fill white +opaque black -blur 0x1 -level 50,100% \) \
-alpha off -compose copy_opacity -composite original_trans2.png



convert original.png \( -clone 0 -fill white +opaque black -blur 0x0.5 -level 50,100% \) \
-alpha off -compose copy_opacity -composite original_trans2.png



If on Windows, then try

convert original.png ( -clone 0 -fill white +opaque black -blur 0x0.5 -level "50,100%%" ) ^
-alpha off -compose copy_opacity -composite original_trans2.png

see http://www.imagemagick.org/Usage/api/ for difference between running IM on Windows and Unix/Mac

Hopefully a Windows user will correct any mistakes I have above.

Note the \ or ^ at the end of the first line, is a line continuation for unix and windows, respectively. You can remove it if you put everthing on one line.

Parenthesis in Unix/Mac must be escaped with \, but they don't need escaping in Windows.


You have not identified your platform nor your version of IM, so it is hard to help as your IM may be old and not support some of these things.

If the above works with one image, then we can look into how you can process multiple image with a loop or with mogrify.

[Art60]

I don't understand your usage of %~ in the following:
convert "%~s1" -transparent black "%~dpn1_x.png"

%1 is the first parameter to a DOS/Windows batch file. Since a while (may be W98), some extensions (~) make batch files easier to write :
So we now got s=short, d=drive, p=path, n=name, e=extension for each part of a filename one may pass as a parameter.

IM doesn't seem to like long names as input file so I used %~s1 to tell it to take the short (8.3) name of 1st batch command line parameter.
If you call my batch file with D:\ImageMagick\thelongfilename.png as a parameter, %~s1 value will be some kind of D:\IMAGE~1\THELON~1.PNG thing.

%~dpn1 tells to produce drive+path+name from 1st arg, so it removes the extension. Then I add _x.png to form the full output filename.
So, if you call my batch file with D:\ImageMagick\thelongfilename.png as a parameter, %~dpn1 value will be D:\ImageMagick\thelongfilename
Then, %~dpn1_x.png value will be D:\ImageMagick\thelongfilename_x.png

You have not identified your platform nor your version of IM, so it is hard to help as your IM may be old and not support some of these things.

I'm running Windows XP SP3 and I downloaded IM the same day I asked for some help, so I think it was the V6.6.4-10 yet

If the above works with one image, then we can look into how you can process multiple image with a loop or with mogrify.

I don't need to make batch conversion of a bunch of files.
The batch file I wrote (with the %~dnp1 thing) should be enough for my software to transform the black background to transparent for each file I get from the freesound website at the time I need to draw it onto my application display. I just have to call something like black2x.bat thepicture.png and I get thepicture_x.png. as easy as this.

Thank you again for your answer, I'll try it tomorrow. (not the same home/work machines)

All the best,
Art.

[fmw42]

Sorry, Windows batch files are out of my domain. Hopefully one of the Windows experts can help you further with translating my command line and your bat file.

[Art60]

Hi,

So I put your command line in a Windows batch file:

Code: Select all

convert "%~s1" ( -clone 0 -fill white +opaque black -blur 0x0.5 -level "50,100%%" ) -alpha off -compose copy_opacity -composite "%~dpn1_x.png"

And it does work:

Original pic


black -> transparent


your command line

Black edges are less visible, thank you very much

But this method adds blur and I'm pretty sure HSL tweaking would do the trick.
I'll be working on that as a second approach.

Thank you again.

All the best,
Art

[Art60]

To All :

Thank you.

I finally found it was very easy to transform darkness into transparency with a single and simple command line parameter : -alpha Copy
My equivalant command line is now this one :

Code: Select all

convert original.png -alpha Copy NiceTransparent.png

Really nice job, isn't it?

Thank you once again!

All the very best,
Art.

[anthony]

It is close but not perfect.

-alpha copy will use the greyscale of the image, which with the colors may not be quite right.

Try extracting a lumince from HSL instead.

But if you are happy with the result. Fair enough.

[Art60]

GreyScale is not Luminance?
I didn't know.

Yes, I thought my picture was good enough

So I'm may dig a little further, may be not right now, though.

Thx for enlighting this.

Art.