[Image-SIG] What does the 'F' mode mean in PIL?

Christopher Barker Chris.Barker at noaa.gov
Fri Jun 17 21:30:08 CEST 2011

A followup, trying to do something real

It seems that the raw decoder doesn't understand all the modes listed in:


indeed, hardly any of them ('F' only for the "floating point modes")

Enclosed is some sample code I've got that attempts to convert a 
unsigned 16 bit grayscale image to RGB. I can't get it to do anything, 
as I get:

ValueError: unknown raw mode

for any floating point mode other than 'F'

What am I missing here?


Christopher Barker wrote:
> Hi folks,
> I've been looking through the docs on File Decoders (or, in my case for 
> decoding binary data with fromstring() or frombuffer()):
> http://www.pythonware.com/library/pil/handbook/decoder.htm
> With the raw decoder, most of the modes are clear to me, but the 'F' 
> mode has me confused:
> What is it really storing, working with?
> - one floating point value per pixel?
> - one floating point value per colorband?
>   - but then, how are they arranged?
> - Is it single precision (32 bit) floats only?
> What does it mean if you use one of the modified 'F' modes, like 
> "F;16N"? I know what an 16 bit native unsigned integer is, but I don't 
> understand:
>  - What happens if I do fromstring() with a mode f "F;16N" ?
>   - is the data converted to a float (32bit)?
>     - if so, do you just lose information if you have higher precision 
> types being passed in?
>   - is the raw data stored in 16 bit (two byte) chunks?
>  - How is 'F' mode interpreted if you convert to, say 'RGB' mode?
>    -grayscale?
>    - something else?
> Any enlightenment would be great.
> -Chris

Christopher Barker, Ph.D.

Emergency Response Division
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Chris.Barker at noaa.gov
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