If the scanned paper has been clipped to pure white, then dithering might not be beneficial. Of course, the file size increases with dithering. 565 have been upconverted to 888 with Photoshop, since PNG doesn't support 16-bit depth natively. Here is a comparison of the software tools I have at hand.
Using the same quantization for all channels (555) would reduce appearance of false color. It doesn't add noise where the color matches exactly, as in pure white. This noise shaper could be the best balance between image quality and file size. The command allows to dial in variable amount of quantization per channel, optionally with ordered dither. However, it can be set up to do simple, repetitive tasks in batch. If somebody knows a tool, which allows batch conversion to RGB565, please let me know!Another option is ImageMagick. Irfan doesn't support configuring PNG prediction.
PNG gives maximum compression on true 16-bit color with prediction (sub/paeth) switched off, because after prediction the color depth expands. You get nasty color banding on scanned documents where large areas are close to white.
What application requires you to use 16-bit color? This was mainly used with older video games in the 1990s to save video memory. PS will redistribute the levels from true black to white upon opening and the contrast/brightness will stay roughly the same, but the color will no longer be precisely quantized to 565. The BMP files saved by Photoshop are true 16-bit, and will exhibit loss of brightness if opened with IrfanView. Photoshop doesn't support dithering in this process, although you could add noise beforehand, less to the green channel, within the same Action. Photoshop can save 16-bit BMP files, which can be converted to another format later if desired.
PHOTOSHOP CONVERT IMAGE TO 16 BIT PRO
You could do this in Photoshop CS2 (freely available) or Paint Shop Pro 9 by recording an Action or Script respectively.
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