Don't be scared, it's not too extensive, though a bit technical!

I was going throw one of the 900/950 FAQ pages on Nikon's European site, when I found this question:

Q - "Why are Black and White (monochrome) files the same size as colour image files?"

A - "The answer has to do with the difference in the way that the eye and brain respond to colour and monochrome images. The monochrome information from a scene is far more important to survival (danger sensing, food identification etc,) than colour. To accommodate this (natural) requirement there are four types of sensor in the eye which can distinguish red, green, blue and monochrome details. The information received by the brain shows a reduction in the less important colour information and an increase in the essential monochrome detail. The Coolpix 900 black and white image mimics the human visual system and recognises that monochrome images should carry more detail than an equivalent colour image. When taking a picture in Black and White mode, the camera derives a high quality image monochrome image from the RGB colour planes and reduces the compression ratio from < to 1/3 to produce a black and white picture of the highest possible standard."

Now, supposing your digital camera had a 3 x 1 pixel CCD, most people would likely believe its design should be something like this:

RGB RGB RGB Where each RGB sensor is a pixel RGB RGB RGB RGB RGB RGB

when what the design really looks, is something like this:

RGBRGBRGB Where each R, G or B sensor is a pixel. GBRGBRGBR BRGBRGBRB

Of course the manufacturer will declare this to be a 9 x 3 pixel CCD for marketing purposes. With this kind of design what you get is 1/3 of the picture's colour information (for the declared resolution) being the remainder 2/3 interpolated from nearby sensors.

The answer on the FAQ page leads me to believe that the CCD's sensors can somehow get some of the extra monochrome information that is lost by the interpolation of the three color planes.

IS THIS TRUE? Or did I miss something?

-- Duarte Bruno (duarte@dgsi.pt), August 12, 1999

Answers

Duarte, have a look at http://www.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=00107s

and see what you think. With that layout of colored pixels, you get virtual pixels formed "in between" each "Block" of 4 pels(a pixel), both vertically & horizontally, which seem to form a pixel. This means that you get 9 pixels from what seems to be a 4 pixel aray composed of only 16 ccd elements.

What this means is that you can get 1280x960 pixels from a 640x480 - 4 Pel per pixel array. Each of the physical 640x480 pixels are composed of a 2x2 array of 4 color filtered CCD sensors. A new virtual pixel is formed "overlapping" the pixels by borrowing an "inside" column or row of 2 pels from each of 2 adjacent 4 pel pixels groups to form a virtual pixel "between" the apparent hardware filtered pixels. Since this is done in both the vertical and the horizontal you get 4 X's as many pixels as the aray seems to contain. If you check the math this fits with th manufacturers claims of how many elements are in the ccd. :-)

For instance, a 640x480 array of pixels composed of a 2x2 array of bb4 color filtered pels each, actually contains 4 x 640 x 480 ccd sensors or 1,228,800 sensors. Just the number you'd expect if it's sold as a 1280x960 pixel unit which most people would "assume" has 1280x960 pixel ccd sensors or 1,228,800 pixels... Cute HUH? :-) This method allows them cheat a bit, but provide a technically accurately speced product.

You have to follow that link to get the text version of the pel layouts to form the pixels. I'm not trying to format it again. Twice was enough! :-)

-- Gerald Payne (gmp@francorp.francomm.com), August 12, 1999.

Thanks Gerald. About CCD design I was far from close to the way the sensors are arranged and interpreted and I've learned a lot of useful information.

I still don't know about B&W pictures but I'm suspicious that it will depend on each camera's algorithms. Right now I can't come up with a way of sorting that out just by lokking at various sizes of images from the Image Resource's samples.

I would really have to get a camera to perform this simple test: get a highly detailed picture to shoot, take the same shot in full color and in B&W, convert the colour one to B&W, compare the two for detail

-- Duarte Bruno (duarte@dgsi.pt), August 13, 1999.

The review here of the CoolPix 950 doesn't compare resolution of colour versus B&W, but it does claim that B&W pictures take less storage space (indicating that colour information is lost, with the same resolution). As I shoot in B&W, I would prefer that colour and B&W images took the same storage space, indicating that B&W has a higher resolution.

If anyone has a camera with a B&W facility, perhaps they could do a resolution test, comparing the two modes?

-- Alan Gibson (Alan.Gibson@technologist.com), August 13, 1999.