Re: [Dvipng] accessing alpha channel

[Bob McElrath]

I use the following sequence of commands using python's Imaging library:

    im = Image.open(os.path.join(workingDir, imname))
    im2 = Image.new('RGBA', (im.size[0], im.size[1]), (255,255,255))
    im2.paste(im, (0, 0))

To explicitly create an alpha channel I use:
    alpha = ImageChops.invert(im.convert('L'))
    im = im.putalpha(alpha)

This creates a PNG with a proper alpha channel.  I would VERY much like
it if dvipng did this for me.  I would love it if you could hack it in
to dvipng.

The code is in latexwiki http://mcelrath.org/Notes/LatexWiki in case I
have omitted some important step in the above code.

John Hunter address@hidden wrote:
> 
> I would like to overlay dvipng rasters over an arbitrary background,
> not just a solid color.  My first thought was that one could extract
> an alpha mask from the dvipng output by having dvipng blend black and
> white and working out what the alpha at each pixel must have been
> using the formula (I'm using the red channel but values are the same
> for blue and green).  First I run dvipng with
> 
>   dvipng -bg Transparent -fg 'rgb 0.0 0.0 0.0' -D 100 -T tight -o outfile.png 
> somefile.dvi
> 
> 
>   red = alpha*red_foreground + (1-alpha)*red_background
> 
> 
> Since the foreground is black (0) and the background is white (1) this
> reduces to red = 1-alpha or 
> 
>   alpha = 1-red
> 
> 
> This gives be an MxN arrays of an alpha mask, and I then blend the
> dvipng raster over an arbitrary background.  
> 
> The problem is that the small rasters blended this way don't come out
> as nicely as the ones rendered by dvipng assuming a constant
> background.  So I must be making some error in my logic.  At first I
> thought I might not be handling gamma properly, but I'm using gamma=1
> so I don't have any special correction for that AFAIK.  I also
> wondered if I'm using the wrong blending formula -- eg is dvipng using
> premultiplied alpha or some other formula to blend.
> 
> So my questions are
> 
>  * would it be possible to modify dvipng to have an option to simply
>    return an alpha mask so we could blend dvi rasters over arbitrary
>    backgrounds?
> 
>  * is there a way to calculate this mask from the output of dvipng as
>    it stands?
> 
> Thanks!
> 
> JDH
> 
> 
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--
Cheers,
Bob McElrath [Univ. of California at Davis, Department of Physics]

    Censorship is never over for those who have experienced it. It is a brand
    on the imagination that affects the individual who has suffered it,
    forever.  -- Noam Chomsky

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