Black and White JPEG Files

[RJ Emery]

On my W98 system, I have a very large (3 MB) JPEG file. The image is True Gray (256 shades). I thought to cut down its size, I could decrease the number of colors and make it a black and white file (1 bit instead of 8 bits per pixel). To my surprise, the resulting file was always larger by a substantial amount. The image itself is an ink drawing of a map, a prime candidate for making it b&w.

I used Paint Shop Pro version 7 and IrfanView 3.80 to attempt the reduction.

1) Should not a conversion from 256 colors to 2 colors result in a smaller JPEG file?

2) How might I do it using PSP7 or IrfanView?

3) What freeware tools (for W98) would be available to effect such a conversion with a corresponding reduction in file size?

[LoisWakeman]

Originally Posted by RJ Emery

1) Should not a conversion from 256 colors to 2 colors result in a smaller JPEG file?

No: because of the way the JPEG algorithm works best on continuous tones: JPEGS are basically 16 million colours and the greyscale is a bit of a fudge: it's not a palette you can edit and save like in a GIF for instance.

I wrote a primer here: http://lois.co.uk/web/articles/web-images.shtml about the different web file types.

You will probably get a smaller file if you convert to a limited palette greyscale GIF or PNG, and of course you can compress the JPEG.
(I have found that line drawings often look very bitty if taken right down to b/w: better to leave some greys to smooth the lines.)

2) How might I do it using PSP7 or IrfanView?

Either will allow conversion to GIF, neither will allow editing and saving a <256 colour palette in a JPEG.

3) What freeware tools (for W98) would be available to effect such a conversion with a corresponding reduction in file size?

None that I know about!