Re: Merging two images

Bob Friesenhahn schrieb:
> On Thu, 29 Oct 2009, Felix Saphir wrote:
>>
>> The next thing for me is to extract parts of the resulting image
>> (rectangular and polygon shapes), and I think, it's going to be an
>> interesting journey :)
> 
> That sounds fun.  Since raster images are rectangular, you will need to 
> do something with the opacity channel to make the hidden pixels 
> transparent, and perhaps by using composition.  There is clip mask 
> functionality in GraphicsMagick which probably could be used.  There is 
> also a poorly documented functionality in which a black/white image can 
> be used to mask out pixels from being changed by an image processing 
> operation, but that only works if the current image is changed, and not 
> for algorithms which produce a new image.

Yes, thought about that as well, and just to document the black/white 
mask algorithm a bit for others reading this mailing list:

1) define two colors, eg. "Magick::Color keep( MaxRGB, MaxRGB, MaxRGB )" 
and "Magick::Color discard( 0, 0, 0 )"

2) set the mask's fill and stroke color to "keep" background color to 
"discard" and draw the shape to the mask

3) set the original image's matte to true and matte color to "keep"

4) compose the mask onto the original image using the 
Magick::CopyOpacityCompositeOp algorithm

This will probably not work with all image types, comments and 
enhancements are welcome!

Anyway, since I need a white background around each extracted shape, I 
use a different approach: The shape is drawn in black on a white 
background, and then composed over the original image with 
Magick::LightenCompositeOp as algorithm. That way, everything outside my 
shape becomes very bright (i.e. white), while nothing is changed on the 
inside.

Or is there a better way to do this?

Regards,
Felix

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