splitcrop and transparency

[Aezer]

Hello,
I want to crop an image into a mosaic of many tiles of the same size. My problem is when a tile exceeds the width (or the height) of the image of origin, I want the tile to be completed with transparency (preserving the fixed size).

Up to now i use the method 'excerpt'. For exemple the code below:

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image.excerpt(-20,-75,256,256)

This code return a tile of 256x256 but fill the unknown pixels with the same as those of the edge of the image. I want the unknown pixels to be transparent.
What is the best way to do this?

Thx (and sorry if my english isn't right)

[snibgo]

At the command line, you might use "-virtual-pixel=none". I don't know the equivalent in whatever language you are using.

[Aezer]

That works perfectly,
Using RMagick:

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img.virtual_pixel_method = Magick::TransparentVirtualPixelMethod

Thank you !