RE: Lossy, smaller JPG files

Subject: RE: Lossy, smaller JPG files
From: Chris Gooch <chris -dot- gooch -at- rocketmail -dot- com>
To: "techwr-l -at- lists -dot- techwr-l -dot- com" <techwr-l -at- lists -dot- techwr-l -dot- com>
Date: Fri, 16 Mar 2012 11:57:48 +0000 (GMT)

Digital cameras and other sources of digital images have had such an
enormous jump in resolution that they exceed the capability of most common
display devices. If you try to display the multi-megapixel image captured by
a modern high-end digital camera on a laptop, or even on a high-resolution
wide-screen monitor, you'll be able to see only a small portion of the
image. These cameras now save images both in this super-megapixel "raw"
format, and in JPGs that "normal" PCs and monitors can handle. 


Actually there's more to it than that ;-)

RAW format just encodes the data from the camera sensors without
any processing in the camera, basically so you can do that yourself
later. It is also higher bit depth (not resolution), i.e., there is more information
per-pixel. This often means there is more dynamic range in the raw data
than can be displayed on a computer screen which, like JPEG, only has
0-255 brightness values in R, G and B (i.e., 8 bits - raw can be 12 or 14 bits).
Also the raw format doesn't impose a colour space, gamma value or
do white balancing or other tone mapping.

So it's not really to do with resolution in terms of number of pixels, more like
the fidelity (depth, range) of the pixel information (although RAW isn't
true High Dynamic Range, you need .exr or .hdr for that which store 16, 24
or 32 bit floating point information per pixel and thus contain many f-stops
worth of dynamic range).

The way to think of it is like RAW is the exposed negative, and you need
to process the film to get a 'print' (something you can display on a screen).
Digital camera can do this processing themselves, or output RAW so you
can do it in your virtual dark-room (i.e.,your favourite photo processing

There's lots of fascinating reading to be had on the subjects of colour spaces,
white balance, high dynamic range and tone mapping if you get into
this stuff  ;-)    (relevant to both digital photography and computer graphics).

There is such a thing as loss-less JPEG compression by the way, but it's not
always clear what packages support it, and if they do, how clear it is to the
user when the compression is loss-less and when it isn't). So I normally
use PNG if I have the choice, for technical writing tasks (certainly for things
like screen captures).


Christopher Gooch,
Technical Writer and Software Developer,
Sheffield, UK.

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