How to estimate size of a Jpeg file before saving it with a certain quality factor?

Question

I'v got a bitmap 24bits, I am writing application in c++, MFC, I am using libjpeg for encoding the bitmap into jpeg file 24bits.

When this bitmap's width is M, and height is N.

How to estimate jpeg file size before saving it with certain quality factor N (0-100).

How can I do it ? Many thanks!

Is it possible to do this?

For example. I want to implement a slide bar, which represent save a current bitmap with certain quality factor N. A label is beside it. shows the approximate file size when decode the bitmap with this quality factor. When user move the slide bar. He can have a approximate preview of the filesize of the tobe saved jpeg file.

Answer 1

It also depends on the compression you are using and to be more specific how many bits per color pixel are you using.

The quality factor wont help you here as a quality factor of 100 can range (in most cases) from 6 bits per color pixel to ~10 bits per color pixel, maybe even more (Not sure).

so once you know that its really straight forward from there..

Answer 2

In libjpeg, you can write a custom destination manager that doesn't actually call fwrite, but just counts the number of bytes written.

Start with the stdio destination manager in jdatadst.c, and have a look at the documentation in libjpeg.doc.

Your init_destination and term_destination methods will be very minimal (just alloc/dealloc), and your empty_output_buffer method will do the actual counting. Once you have completed the JPEG writing, you'll have to read the count value out of your custom structure. Make sure you do this before term_destination is called.