[magick-users] average ignoring transparency

[Karl Billeter]

On Mon, Jan 19, 2009 at 02:24:50PM +1000, Anthony Thyssen wrote:
> > What I'd like to be able to do is to only average parts of images that are
> > not transparent.
..

> Do what everyone else does,  mask the resulting images with the
> original using DstIn operators  (mask using the transparency shape given)

I can see how that would work for averaging two images but not for an
arbitrary number.  Say I had four images, possible overlaps are:

1  2  3  4                    # no overlap, no average 
1+2  1+3  1+4  2+3  2+4  3+4  # sum channels and divide by two
1+2+3  1+2+4  2+3+4           # sum channels and divide by three
1+2+3+4                       # sum channels and divide by four

I can't see how to generalise this with masking but I'm not practised at image
manipulation.


I've tried using PerlMagick to handle pixel data myself

reading in for each image, for each pixel:

    my $idx = $j*$height +$i;  # index into data array
    my @pixel = $im->GetPixel( x=>$i, y=>$j, channel=>'All' );
    my $opacity_weighting = 1 - $pixel[3];
    $data[$idx][0] += $pixel[0] * $opacity_weighting;
    $data[$idx][1] += $pixel[1] * $opacity_weighting;
    $data[$idx][2] += $pixel[2] * $opacity_weighting;
    $data[$idx][3] += $pixel[3];
    $data[$idx][4] += 1 unless $pixel[3] == 1;


writing out for each pixel:

    my $idx = $j*$height + $i;  # index into data array
    my $image_cnt = $data[$idx][4];
    my $image_total = scalar @images_in;
    my @pixel = (0,0,0,1);
    $pixel[0] = $data[$idx][0] / $image_cnt if $image_cnt;
    $pixel[1] = $data[$idx][1] / $image_cnt if $image_cnt;
    $pixel[2] = $data[$idx][2] / $image_cnt if $image_cnt;
    $pixel[3] = $data[$idx][3] / $image_total;
    $image_out->SetPixel(x=>$i,y=>$j,channel=>'All',color=>\@pixel);

And this seems to work but I lose my transparency...

> Average like most mathematicaly operators do not define what they should
> do with regards to transparency!


Regards,
Karl