Image Size and Compression.
 

Let's assume I have a 10MP camera
My sensor is say, 3650 X 2740 pixels.
But say I want to create an image at 1825 x 1370 pixels.
How does the camera actually reduce the 5.0MPs to 2.5MPs
Does it choose groups of 4 pixels and somehow average them out to groups
of 1 pixel each?
How does this process differ from compressing the 10MP image by a factor
of 4.
I know that in one case the image SIZE is reduced (as well as the file
size) whereas in the other case, the image SIZE remains the same but the
file size is reduced.
How exactly does each process affect the appearance of say an 8x10 print.
Bob Williams

Image Size and Compression.
 

bobwilliams wrote:
Let's assume I have a 10MP camera
My sensor is say, 3650 X 2740 pixels.
But say I want to create an image at 1825 x 1370 pixels.
How does the camera actually reduce the 5.0MPs to 2.5MPs
Does it choose groups of 4 pixels and somehow average them out to groups
of 1 pixel each?
How does this process differ from compressing the 10MP image by a factor
of 4.
I know that in one case the image SIZE is reduced (as well as the file
size) whereas in the other case, the image SIZE remains the same but the
file size is reduced.
How exactly does each process affect the appearance of say an 8x10 print.
Bob Williams

OOPS!
I meant to say, How does the camera actually reduce the 10MPs to 2.5MPs?
Bob

Image Size and Compression.
 

On 30/07/2010 10:36, bobwilliams wrote:
bobwilliams wrote:
Let's assume I have a 10MP camera
My sensor is say, 3650 X 2740 pixels.
But say I want to create an image at 1825 x 1370 pixels.
How does the camera actually reduce the 5.0MPs to 2.5MPs
Does it choose groups of 4 pixels and somehow average them out to
groups of 1 pixel each?
How does this process differ from compressing the 10MP image by a
factor of 4.
I know that in one case the image SIZE is reduced (as well as the file
size) whereas in the other case, the image SIZE remains the same but
the file size is reduced.
How exactly does each process affect the appearance of say an 8x10 print.
Bob Williams

OOPS!
I meant to say, How does the camera actually reduce the 10MPs to 2.5MPs?
Bob

When using a smaller pixel count, what a given camera does exactly is
hardly documented. Most processing would be built around these
algorithms (in decreasing order or quality and computing power needs):

http://en.wikipedia.org/wiki/Lanczos_resampling
http://en.wikipedia.org/wiki/Bicubic_interpolation
http://en.wikipedia.org/wiki/Bilinear_interpolation
http://en.wikipedia.org/wiki/Nearest..._interpolation

PC software normally uses the first two. Some pre-blurring, and
post-sharpening can be used. This would also be combined with the
demosaicing algorithm.

--
Bertrand

Image Size and Compression.
 

On Fri, 30 Jul 2010 01:25:37 -0700, bobwilliams wrote:

Let's assume I have a 10MP camera
My sensor is say, 3650 X 2740 pixels. But say I want to create an image
at 1825 x 1370 pixels. How does the camera actually reduce the 5.0MPs to
2.5MPs Does it choose groups of 4 pixels and somehow average them out to
groups of 1 pixel each?
How does this process differ from compressing the 10MP image by a factor
of 4.
I know that in one case the image SIZE is reduced (as well as the file
size) whereas in the other case, the image SIZE remains the same but the
file size is reduced.
How exactly does each process affect the appearance of say an 8x10
print. Bob WilliamsÕÅ?€(¹

Why not try it.

You would then know that whatever results you get are real and not
guessed at by someone else.



--
Neil - reverse 'ra' and delete 'l'.

Image Size and Compression.
 

On Fri, 30 Jul 2010 11:03:33 +0200, Ofnuts
wrote:

On 30/07/2010 10:36, bobwilliams wrote:
bobwilliams wrote:
Let's assume I have a 10MP camera
My sensor is say, 3650 X 2740 pixels.
But say I want to create an image at 1825 x 1370 pixels.
How does the camera actually reduce the 5.0MPs to 2.5MPs
Does it choose groups of 4 pixels and somehow average them out to
groups of 1 pixel each?
How does this process differ from compressing the 10MP image by a
factor of 4.
I know that in one case the image SIZE is reduced (as well as the file
size) whereas in the other case, the image SIZE remains the same but
the file size is reduced.
How exactly does each process affect the appearance of say an 8x10 print.
Bob Williams

OOPS!
I meant to say, How does the camera actually reduce the 10MPs to 2.5MPs?
Bob

When using a smaller pixel count, what a given camera does exactly is
hardly documented. Most processing would be built around these
algorithms (in decreasing order or quality and computing power needs):

http://en.wikipedia.org/wiki/Lanczos_resampling
http://en.wikipedia.org/wiki/Bicubic_interpolation
http://en.wikipedia.org/wiki/Bilinear_interpolation
http://en.wikipedia.org/wiki/Nearest..._interpolation

PC software normally uses the first two. Some pre-blurring, and
post-sharpening can be used. This would also be combined with the
demosaicing algorithm.

There are dozens more resampling algorithms than that available in good
computer software. Like Bell, Spline, Pyramid, Mitchell, Vector, Triangle,
Hermite, Catmull-Rom, and all their various incarnations. Bicubic being the
most often used in-camera algorithm due to its speed (vs. poorer
detail-retaining performance) that is required for fast in-camera
processing. I know of NO camera that would ever use the processing
intensive Lanczos algorithms. That's just wishful dreaming on your part.
Nor do I know of any that would use the rudimentary Bilinear or Nearest
Neighbor. Those are just too crude for any in-camera processing.

Please only offer advice and opinions on things that you actually know
about. It's so tedious having to continually correct you lousy ****-head
pretend-photographer role-playing trolls.

Image Size and Compression.
 

On 30/07/2010 11:46, BFD wrote:

Please only offer advice and opinions on things that you actually know
about. It's so tedious having to continually correct you lousy ****-head
pretend-photographer role-playing trolls.

This was just a ploy to trick you into sharing your Immense Knowledge
with us mere mortals, O Grand Master Of All Things Photographic...
--
Bertrand

Image Size and Compression.
 

On 30/07/2010 09:25, bobwilliams wrote:
Let's assume I have a 10MP camera
My sensor is say, 3650 X 2740 pixels.
But say I want to create an image at 1825 x 1370 pixels.
How does the camera actually reduce the 5.0MPs to 2.5MPs
Does it choose groups of 4 pixels and somehow average them out to groups
of 1 pixel each?

It does (or rather should do) something a little bit more sophisticated
than a simple average. It has to low pass filter the image to downsample
and avoid producing Moire fringe aliasing artefacts.

How does this process differ from compressing the 10MP image by a factor
of 4.

Critically the top half of the high frequency components present in the
orginal image are lost forever when you downsample to a half size one.
The information content and size is reduced accordingly.

I know that in one case the image SIZE is reduced (as well as the file
size) whereas in the other case, the image SIZE remains the same but the
file size is reduced.
How exactly does each process affect the appearance of say an 8x10 print.
Bob Williams

The finest visible detail in the 10Mpixel image will be about 1/300"
across whereas in the 2.5Mpixel image it will be 1/150".

For my money the higher resolution image using higher compression will
almost always beat the lower resolution less compressed image. There can
be exceptions and unless you are absolutely certain you will never need
the extra pixels or you are running out of media space there is little
or no advantage in decreasing image size in the camera.

Regards,
Martin Brown

Image Size and Compression.
 

On Fri, 30 Jul 2010 12:59:32 +0100, Martin Brown
wrote:


For my money the higher resolution image using higher compression will
almost always beat the lower resolution less compressed image. There can
be exceptions and unless you are absolutely certain you will never need
the extra pixels or you are running out of media space there is little
or no advantage in decreasing image size in the camera.

Regards,
Martin Brown

Showing how little you know.

If using higher ISO's with more noise, it can be advantageous to use
in-camera downsampling. As this will average-out the noise from the RAW
sensor data.

Image Size and Compression.
 

Outing Trolls is FUN! wrote:
On Fri, 30 Jul 2010 12:59:32 +0100, Martin Brown
wrote:

For my money the higher resolution image using higher compression will
almost always beat the lower resolution less compressed image. There can
be exceptions and unless you are absolutely certain you will never need
the extra pixels or you are running out of media space there is little
or no advantage in decreasing image size in the camera.

Regards,
Martin Brown

Showing how little you know.

If using higher ISO's with more noise, it can be advantageous to use
in-camera downsampling. As this will average-out the noise from the RAW
sensor data.

It would be more "advantageous" to retain the original data
and use a superior noise reduction algorithm later.

You can look up noise reduction algorithms on your own time
if you think averaging is a good one.

BugBear

Image Size and Compression.
 

On Fri, 30 Jul 2010 13:54:47 +0100, bugbear
wrote:

Outing Trolls is FUN! wrote:
On Fri, 30 Jul 2010 12:59:32 +0100, Martin Brown
wrote:

For my money the higher resolution image using higher compression will
almost always beat the lower resolution less compressed image. There can
be exceptions and unless you are absolutely certain you will never need
the extra pixels or you are running out of media space there is little
or no advantage in decreasing image size in the camera.

Regards,
Martin Brown

Showing how little you know.

If using higher ISO's with more noise, it can be advantageous to use
in-camera downsampling. As this will average-out the noise from the RAW
sensor data.

It would be more "advantageous" to retain the original data
and use a superior noise reduction algorithm later.


Of course it would. But that was not the question nor possible answer. I
purposely set all my cameras to lowest contrast (retains fullest dynamic
range in the JPG output), lowest noise-reduction, and lowest sharpening
settings so that I may do that better on the computer. If available (as in
CHDK cameras) I will use a live-view RGB histogram to determine if any one
or more of the color channels are also out of whack and will also adjust
those accordingly so that one will not be blown-out before another.

However, it can be even better to use a RAW-Averaging feature as is
available in all CHDK P&S cameras' in-camera processing to provide
completely noise-free images at ISO800, 1600, and higher.

You speak as if others don't know more than you ever will.