Enlarge already big jpegs using less memory

Question

I need to enlarge already big jpegs, they are used on printing so they need to be really big 300PPI files. The resulting image will be too big to be fully hold in memory. What i thought was something like breaking the original image into small strips, enlarge each one of them separatedly and go writing it in the output file(another jpeg), never keeping the final image fully on menoru. I've read about lossless operations on jpegs it seems the way to go(create a file with the strip and copy the mcus, huffman tables and quantizatuon tables to the final file), also read something about abbreviated streams on java. What is a good way to do this?

Answer 1

Your best bet is to leave the JPEGs alone and have the printing software scale the output to the device.

If you really want to double the size,you could use subsampling. Just double the Y component in each direction and change the sampling for Cb and Cr while leaving the data alone.

You could also do as you say, and recompress in strips of MCUs.