The initial picture is encoded with 24 bits depth. I mean 8 bits for Red, 8 bits for Green, 8 bits for Blue. This gives 2 power 24 different colors ... A lot of colors ! Too much colors for my application.
So I would like to shrink number of colors.
The way to do that is to encode Red with 5 bits, Green with 6 and Blue with 5 bits. That's the reason it's called RGB565.
I am sure that Photoshop CS3 can do that very well but I don't find where it is !
Do U follow me ?
so basically what you want is to get a smaller file? well then you should use File->Save for Web&Devices and save as a jpg or png-8 as the bmp file format tends to make huge files. If you have to save to the .bmp file format then when saving the file, click on Advanced Modes in the BMP Options dialog box and chose R5 G6 R5
I am still using BMP 565 for a client, though I'd rather use PNGs. Some clients don't want to go near any format that has intellectual property issues, though I'd love to know if anyone is being sued for using a PNG or a JPEG these days. Unfortunately there is no support for BMPs in Save For Web.
I can't remember with CS3, so it might not be an option, but with CS4 and CS5, here's what you do:
1. Save As...
2. Select BMP format in the Save As... dialog box
3. It should give you a list of normal formats, plus a button for "Advanced Modes"
4. Pick "R5, G6, B5" mode
5. Click OK and you're done