Legacy ImageMagick Discussions Archive

Hello,

Is there a way to read/uncompress Blobs row-by-row?
I use Magick++ with pre-loaded Blobs in memory, and something like this:
Magick::Image img;
try { img.read(*pblob); }

However, I need only a few rows at a time, but from many different images - uncompressing all of them takes quite a time, and if possible I'd like to optimize this by reading only a few rows from each images.

Thanks,
igy137

To access a few image rows at a time you need to work directly with pixels. When reading a blob, the entire blob is read and the entire image is created in memory or on disk.

Thanks for the answers!
What a pity...
BTW, By traicing into the sources I saw, that for example gif is decompressed row-by-row (maybe I'm wrong, I just take a quick look at it). What happens if I call read blob from a low priority thread and try to access the image row-by-row from another thread? Is there some mechanism which prevents access of uncompressed parts, or any indication (I use getConstPixels(..) call)

D'oh. Forgot to mention the streaming interface which may do exactly what you are asking for. See http://www.imagemagick.org/script/archi ... php#stream.

Thanks, I take a look at it!

This really looks good.
Unfortunately as I was not able to figure out how could I use this from Magick++ (if possible at all).
Can you help me, please? (It's also ok to use MagickCore if it's not possible with Magick++).
Thanks again for the hint!

You can use MagickCore from Magick++ by prefixing the methods with MagickLib:: (e.g. MagickLib::ReadStream()). Have fun.

Finally, I got this working ))
I still have some questions, though:
Is this thingy thread-safe? Can I use multiple threads for simultaneous extraction of different images?
Is there any way to pass some data to the callback function receiving the pixels?

Thread safe? Sure, discussed indirectly in the ImageMagick architecture document. Every method in ImageMagick is thread safe except for Acquire/Get/SetImagePixels().

Streaming has a client_data member but apparently there is no method to set it. We will get a patch in ImageMagick 6.3.3 to correct this problem. Most likely SetStreamClientData() and GetStreamClientData().

Great! Although, SetImagePixels is actually called by the decompressor (at least by ReadGIFImage) before I get the stream callback

We keep things thread safe internally. We're talking about the public API being thread safe (everything but Acquire/Get/SetPixels()). The streaming interface is low level and most of the stream methods are defined in stream-private.h which is not part of the public API. So we'll add SetStreamInfoClientData()/GetStreamInfoClientData() but you will need to include stream-private.h and assume the risk of using a private interface (private methods are subject to signature change although unlikely).