Turn PDF into grayscale

[Dariusz J. Garbowski]

On 06/27/2006 12:23 AM, Sundar Nagarajan wrote:
> Dariusz J. Garbowski wrote:
>> On 06/26/2006 11:11 PM, Sundar Nagarajan wrote:
>>> Pupeno wrote:
>>>> Hello,
>>>> I have some color PDFs I'd like to turn into grayscale, any ideas how ?
>>>> I have tried printing to PDF (the pdf itself) and setting it to 
>>>> grayscale, but the PDF ends up in color anyway.
>>>> Thank you.
>>>>
>>>
>>> You can do this quite simply at the command line using the 
>>> Imagemagick tools. Install imagemagick ('apt-get install imagemagick' 
>>> should do the trick).
>>>
>>> to convert a file named color.pdf to grayscale and store the output 
>>> in a file named gray.pdf:
>>>
>>> convert -colorspace GRAY color.pdf gray.pdf
>>>
>>> The only drawback I see is that this always seems to choose a gray 
>>> colorspace with 256 colors (while the original colorspace may have 
>>> (e.g.) 64K colors. Maybe someone else can comment whether the GRAY 
>>> colorspace is always 8-bit.
>>
>> This is simple: gray colour is made of all three RGB channels having 
>> the same value. Thus you have:
>>
>>   R   G   B  shade of gray
>>   0   0   0  black
>>   1   1   1  almost black
>>   2   2   2  almost black
>> ...
>> 127 127 127  gray
>> 128 128 128  a bit lighter gray
>> ...
>> 255 255 255  white
>>
>> what makes 256 colour space (or "gray space" ;-) This can only improve 
>> if you have more bits-per-channel. Notice that 24 bit colour jpg is 
>> only 8 bit per channel (3 channels * 8 bit = 24 bit).
>>
>> Regards,
>> Dariusz
> 
> Ahh.. that is a good explanation.. I should have done a little more 
> thinking myself :-(
> 
> So the color reduction algorithm maps an RGB color to the "nearest" RGB 
> subset having the same values for R, G and B? Then that single identical 
> value is the "gray color"?


See my other post in this thread explaining this (which I sent on 
06/27/2006 01:00 AM).

Regards,
Dariusz

		
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