CODEWITHSUNDEEP
c++jpeglarge-filescrop
briskly
briskly

Reputation: 193

Cropping large jpeg

There is a task, to write a programm that will be crope a JPEG files. But the problem is that some jpeg files has large sizes - hundreds of MegaBytes. So the question: Is it possible to crop a jpeg file, but without loading all file to the RAM, using something like fseek(), and decoding only the parts that needed.

Is that possible? If yes, maybe there is some libraries do the same.

Upd. All this will be used for the deep zoom technology. So when deep zoom will asking for a file, this program will give it, but this should be in real time

Upvotes: 0

Views: 456

Answers (1)

Mark Ransom
Mark Ransom

Reputation: 308081

There are two ways to accomplish this.

The first is lossless cropping, where you don't decode the file all the way but work with the 8x8 DCT blocks. You'll need to use a library that has this capability, and it places some restrictions on the cropping ability. You can't crop to a boundary that isn't on the DCT square, which limits you to multiples of 8 or 16 depending on the subsampling in the file.

The second way is to use a library that allows you to read and write one line at a time. I know that the IJG library can do this, and probably others as well. This is the easy way, but the downside is that the image goes through a decompression/recompression pass and will lose quality and/or be larger.

Upvotes: 5

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