bitdepth of CHDK RAW files (powershot)

Hello.

Does anyone know what the bitdepth is of RAW files shot with a
powershot S2 or S3 using the CHDK firmware?
Also, what is the best way to convert such RAW files to DNG or TIF
in a way that yields the same resolution as JPG files created by the
camera? I've noticed that the DNG conversion tool suggested by the
CHDK FAQ seems to yield DNG files that have a lower resolution than
the JPG files created by the camera.

kind regards and thanks in advance for any suggestions, Niek

On Fri, 05 Oct 2007 23:45:52 -0700, sobriquet wrote:


Hello.

Does anyone know what the bitdepth is of RAW files shot with a
powershot S2 or S3 using the CHDK firmware?

10-bit, non-standard RAW.

So instead of getting 16(million),777,216 possible colors and shades per pixel
(256x256x256), you get 1(billion),073,741,824 colors and shades per pixel
(1024x1024x1024). 8-bits is 0 to 256, 10 bits is 0 to 1024.

Though technically, since there are 4 sensor areas per pixel, that should be
8x8x8x8 and 1024x1024x1024x1024. But since the 2 green sensors are in effect
getting the same luminosity reading they are averaged out. If a color is
changing over one of those pixel boundaries, then they can indeed get
1(trillion),099,511,627,776 possible shades/hues.

You probably won't notice this. :-)

It's times like this where you'd better have an editor with a real 32-bit math
platform to fully use these larger color depths. (FYI: PhotoShop is only 16-bit.
Always has been, always will be.)


Also, what is the best way to convert such RAW files to DNG or TIF
in a way that yields the same resolution as JPG files created by the
camera? I've noticed that the DNG conversion tool suggested by the
CHDK FAQ seems to yield DNG files that have a lower resolution than
the JPG files created by the camera.


I find the easiest way is using RAW Therapee or PhotoLine 32. PhotoLine 32
because after conversion then it's already loaded into my most used editor.
DNG4PS works nice too because it copies the corresponding JPG EXIF info to the
resulting DNG file. DCRaw is also nice, because that works fast as a DOS
command-line utility while also giving you the option of exporting in TIF or PPM
formats too. Lots of command-switch options with that one. I don't seem to find
any problems with color shifts that other experience from their cameras when
using it. It might be the version I am using specifically for the S3 IS.

You won't get the same resolution as the JPG files, you will always get slightly
more, the amount depending on the conversion method/program that you use. Every
camera has a small border of extra pixels on the sensor that is automatically
dumped when converted to JPG in the camera. Most, not all, of the RAW to (other
type) conversion utilities do not truncate this small border of extra image.

See this link
http://scratchpad.wikia.com/wiki/Tal..._image_area.21

You also get more detail out of the RAW data than the JPG file. Much of the
details and color information in the shadows is often lost during the in-camera
JPG conversion. But this is true too of details in general. I found when doing
tests on a high-resolution target, set at an arbitrary distance: if the lines
resolved in the JPG file extinguished at 14 they would extinguish at 15 or 16
for the RAW file. Or if at 16 in the JPG, then 17 or 18 on the RAW. There was
always that little bit extra resolution from the RAW file compared to the JPG.
About the same increase you would get from the JPG files alone when going from a
6 megapixel camera to an 8 or maybe even a 10.

The RAW to JPG algorithms that you can use in external applications will always
be far better than any in-camera ones due to their limited processor speed and
programming. You can't update the interpolation method in your camera, but new
RAW to (other file type) methods are being invented regularly.

Check out the comparison page at RAW Therapee's site for how the various methods
equate. Keeping in mind that none of them in any camera will ever be as good
when going from RAW to JPG as you can do in your computer.

http://www.rawtherapee.com/?page=compare