Pixelate a portion of an image to hide information - ruby

Question

I my application users are able to upload photos. Sometime, I want them to hide some information of the picture, for instance the registration plate of a vehicle, or the personal address of an invoice.
To meet that need I plan to pixelate a portion of the image. How can I pixelate an image in such a way given the coordinates of the area to hide and the size of the area.
I found out how to pixelate (by scaling the image down and up) but how can I only target an area of the image?
The area is specified by two pairs of coordinates (x1, y1, x2, y2), or a pair of coordinates and dimensions (x, y, width, height).

Answer 1

I am at work at the moment so can not test any code. I would see if you could work with -region or else use a mask.
Copy the image and pixelate the whole image create a mask of the area required, cut a hole in the original image with the mask and overlay it over the pixelated image.
You could modify this code ( quite old and could probably be improved on ):
// Get the image size to creat the mask
// This can be done within Imagemagick but as we are using php this is simple.
$size = getimagesize("$input14");
// Create a mask with a round hole
$cmd = " -size {$size[0]}x{$size[1]} xc:none -fill black ".
" -draw \"circle 120,220 120,140\" ";
exec("convert $cmd mask.png");
// Cut out the hole in the top image
$cmd = " -compose Dst_Out mask.png -gravity south $input14 -matte ";
exec("composite $cmd result_dst_out1.png");
// Composite the top image over the bottom image
$cmd = " $input20 result_dst_out1.png -geometry +60-20 -composite ";
exec("convert $cmd output_temp.jpg");
// Crop excess from the image where the bottom image is larger than the top
$cmd = " output_temp.jpg -crop 400x280+60+0 ";
exec("convert $cmd composite_sunflower_hard.jpg ");
// Delete tempory images
unlink("mask.png");
unlink("result_dst_out1.png");
unlink("output_temp.jpg");

Answer 2

Thanks for your answer, Bonzo.
I found a way to achieve what I want with ImageMagick convert command. It's a 3-steps process:
I create a pixelated version of the whole source image.
I then build a mask using the original image (to keep the same size) filled with black (with gamma 0) then I draw blank rectangle where I want unreadable areas.
Then I merge the three images (original, pixelated and mask) in a composite operation.
Here is an example with 2 areas (a et b) pixelated.
convert original.png -scale 10% -scale 1000% pixelated.png
convert original.png -gamma 0 -fill white -draw "rectangle X1a, Y1a, X2a, Y2a" -draw "rectangle X1b, Y1b, X2b, Y2b" mask.png
convert original.png pixelated.png mask.png -composite result.png
It works like a charm. Now I will do it with RMagick.