PHP conversion JPG to PNG and reverse, both JPG images should have same sizes as PNG is lossles format

Question

I am using imagecreatefromjpeg() and imagepng() functions to convert JPG image to PNG. After that I am converting that PNG image back to JPG format with imagecreatefrompng() and imagejpeg() functions. As PNG is lossles format, both JPG images before and after conversions should have the same size, but sizes are different, why?

Thanks.

Answer 1

The sizes will only be the same if:

1. You use the exact same quantization tables.
2. You use the exact same sampling rates.
3. Each component uses the exact same Huffman tables.
4. The number of Huffman tables is the same.
5. The encode and decode create the same APPn and COM markers.