There is a small piece of code that does what I expect it to: 
#!/usr/bin/perl

use Image::Magick;

$ipimage = Image::Magick->new(size=>'640x480');
$ipimage->Read("logo:");

$blob1=$ipimage->ImageToBlob();

$opimage = Image::Magick->new(size=>'640x480');
$opimage->BlobToImage($blob1);

$opimage->Write("t1.jpg");

[download]
It writes a JPEG of the ImageMagick logo. Here is a second piece of code. The only difference is that I have specified an RGB format for the image: 
#!/usr/bin/perl

use Image::Magick;

$ipimage = Image::Magick->new(size=>'640x480');
$ipimage->Set(magick=>'RGB');
$ipimage->Read("logo:");
$blob1=$ipimage->ImageToBlob();

$opimage = Image::Magick->new(size=>'640x480');
$opimage->Set(magick=>'RGB');
$opimage->BlobToImage($blob1);

$opimage->Write("t2.jpg");

[download]
In this case, the output is still a 640x480 JPEG image. However, the icon is a quarter the size, it's repeated in the top left and right quadrants (the lower half of the image being black) and the image colours are wrong.

I have looked for any worked examples of BlobToImage and come up with nothing. What documentation I have found seems to be abstract API definitions, but without enough information to say for sure if I'm calling it correctly. I'd really appreciate someone explaining to me why I get these different outputs, and how to call BlobToImage for an RGB image (since I need to manipulate the individual colour pixels and GetPixel/SetPixel is far too slow) to create the sort of output that the first code example gives.

With thanks

In reply to ImageMagick - why do these snippets act differently? by pete_l

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