Image Size and Compression.
in [Digital Cameras]
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From: bobwilliams on 30 Jul 2010 04:25 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
From: bobwilliams on 30 Jul 2010 04:36 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
From: Ofnuts on 30 Jul 2010 05:03 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-neighbor_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
From: Neil on 30 Jul 2010 05:17 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'.
From: Martin Brown on 30 Jul 2010 07:59
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 |