Text in a raster image is not the same as the same text in a text file. To convert rasterised text to computer text OCR is needed. The quality of current systems isn't that great.
== Degree Students ==
The point of using layers is that you can combine them into one image but still make changes to the individual components. This is a very powerful tool mostly used by graphics experts but it's useful even for some simple tasks.
Read the 5 page paper [[Media:lossless_re-enconfig_of_jpeg_images.pdf|Lossless Re-encoding of JPEG Images Using Block-adaptive Intra Prediction (Matsuda et al, 2008)]]. It's quite complicated but see how much useful information you can extract from it without spending a month learning the math.
= Links =