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Old Sep 14, 2007, 5:26 PM   #1
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I'm considering the purchase of a Cannon G9 Power Shot camera and printed out the specs from the Cannon website. Some of the specs are listed as:

Type: 12.1 Mega pixels - Image Compression: Normal, Fine, and Super Fine – Recording Pixels: Large (4000x3000), Medium 1 (3264x2448, Medium 2 (2592x1944) etc. I assume the large (4000x3000=12,000,000) yields the spec of 12.1 mega pixel. If this is true and the camera is set to Medium 1, does the camera in effect become an 8 mega pixel camera and at Medium 2, a 5 mega pixel camera? Also I don't have a good handle on howImage Compression relates to picture quality.



If the above is not true and the explanation is too long for this forum, please refer me to websites where the clarifications are for a novice.



Thanks . . .
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Old Sep 14, 2007, 5:53 PM   #2
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gstephens wrote:
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... If this is true and the camera is set to Medium 1, does the camera in effect become an 8 mega pixel camera and at Medium 2, a 5 mega pixel camera?
When the camera is set to 'Medium 1', the processor in the camera interpolates the data it has from the 12 million pixels to create an 8 megapixel image.

gstephens wrote:
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Also I don't have a good handle on howImage Compression relates to picture quality.
Each pixel contains the original data, but preserving the data in each and every pixel requires a lot of storage space. JPEG Compression tries to consolidate some of the data in low contast/low detail areas of the image, but as more compression is applied, more contrast and detail are lost.

See http://en.wikipedia.org/wiki/JPEG. There is an example of how varrying amounts of compression affect image quality.
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Old Sep 14, 2007, 5:55 PM   #3
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Here is an article that discusses image file formats and jpeg compression.

JPEG Images: Counting your Losses by Mike Chaney

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Old Sep 20, 2007, 1:35 PM   #4
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Thanks for the replies. After reading the data, I now realize that a novice has no chance of completly understanding compression; however, I believe you have answered my querry to the level of my understanding.

Thanks again and Thanks to the website forthisforum
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Old Sep 20, 2007, 2:38 PM   #5
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gstephens wrote:
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... I now realize that a novice has no chance of completly understanding compression ...
Compression is a Black Art. It's not important for you to understand compression; it's only important for you to understand what it will do to your images. The more you compress an image, the more it will look like a gray sheet of paper; the less you compress it, the more it will look like the scene you took a photo of.
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Old Sep 20, 2007, 3:25 PM   #6
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There are a couple of qualitative things that a photographer should know about JPEG compression. First, it converts your image into YCrCb color space. This converts the image Red Green and Blue data into three other chunks of data -- a greyscale rendition of the image (Y)and two color planes (Cr is a plane that represents the Red/Green continuum and the Cb represents Blue/Yellow.) The reason for this is that we are a lot less sensitive to changes in hue than we are to changes in luminance. So standard JPEG only keeps one Cr and one Cb value for every two Y pixels. Before it even starts "compressing" the image, it has thrown away a third of the pixel data.

Then, JPEG aggressively smoothes the Cr and Cb color channel data and more gently smoothes the Y channel data -- again, because you don't see changes in hue nearly as easily as changes in luminance.So, what is the result of this? First, JPEG data starts to lookkind of "pastelly"as its first sign of being over-compressed -- the color channels get overcompressed first. Pushing that compression further actually leads to posterization -- the colors get so obviously quantized that you can seewhat looks like contour maps in relatively continuous color regions.You will often see it in expanses of sky on JPEG images.

Compression of the luminance channel is generally hard to spot. If you really over-compress the image, you will start to see little squares in the image. These are because the unit of compression in a JPEG image is an 8x8 pixel block. If they get too compressed, you start seeing the individual blocks. Except when you start seeing the little squares, I can't think of any images that I have seen greyscale features disappear from due to overcompression. So the things that I think of as tell-tale signs of overcompression with JPEG are first, the colors starting to look washed out, then second, the colors becoming posterized, and finally, artifacts of small squares appearing in the image. If you select the highest quality JPEG setting for your camera, you will seldom encounter these effects.

If you don't already know it, you should avoid multiple savesin JPEG, because the lossy effects are cumulative. If you're going to edit a photograph, convert the image to a bmp file or the like, and do all your PhotoShop work on that. When you are through with allyour edits, crops, and resizing, save the final image as a JPEG.Since this is sometimes a source of confusion for non-programmers, let me say explicitly that viewing a file does not involve any recompression of the file. You can open it for viewing as many times as you like without degrading the file. It is only when you are editing the file that you need to worry about "generational loss," which is the technical term for what happens when you compress a JPEG file over and over again. FWIW


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