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M ov ing f rom 2 D P oints to 3 D V ol um e s


In the mid-1880s, Neo-Impressionist artist Georges Seurat perfected a revolutionary technique of painting with tiny dots of color. Like Michel Chevrul before him, Seurat recog- nized that from a distance, the eye would naturally blend together tiny dots of primary colors to produce secondary shades. Using tiny brush strokes, Seurat and his contemporaries captured scenes of cityscapes, harbors and people at work and leisure. This technique came to be known as pointillism. Computers use a similar technique to display text and images; however, they work at a much finer scale. Every image portrayed on a com- puter monitor or video screen is composed of many, almost imperceptibly tiny dots, spaced at extremely close intervals. In a 2D picture screen, each dot, or pixel (a word formed from the contraction of picture element) can be defined by its horizontal (x) and vertical (y) screen coordinates. It is also defined by its color value. In color images, each pixel is also assigned its own brightness.


The number of shades that a pixel can take on depends on the computer and the number of bits per pixel (bpp) it is capable of process- ing. Common values range from 8 bpp (28 bits, which translates to 25 6 colors) to 24 bpp (224 bits, or 16,777,216 colors). On an eight-bit gray-scale image, for instance, each pixel would be assigned a value corresponding to a shade of gray, ranging from 0 to 25 5 , where 0 represents black and 25 5 represents white. The number of pixels used to create an image controls its resolution (above right). As more pixels are used, the image can be por- trayed in greater detail, or higher resolution. Resolution is thus initially impacted by the image acquisition system and later, by the image display system.


Resolution in digital image acquisition systems is largely governed by the number of light-sensitive photoreceptor cells, known as photosites, which are used to record an image. These photosites (more commonly referred to


> Pix el resolution. The sharpness and clarity of an im age are affected b y pix el count and the size of the pix els. To increase the num b er of pix els w ithin a  x ed space, pix el size m ust b e reduced. As pix el size ( in w hite) progressively decreases ( left to right) , m ore pix els can b e used to provide greater detail in the im age.


0 1 ,0 0 0 8 0 0 c ol or 6 0 0 4 0 0 x 2 0 0 y 0 0 2 0 0 4 0 0 6 0 0 8 0 0 H orizontal coordinates, x 1 ,0 0 0 0 0 P ix e l 6 0 0 z x 4 0 0 y 2 0 0 2 0 0 4 0 0 6 0 0 8 0 0 H orizontal coordinates, x


> Pix el to vox el. A  at pix el ( left) tak es on a new dim ension w hen the slice on w hich it resides is stack ed w ith other slices to form a volum e ( right) . Adding the z-coordinate of the slice num b er essentially assigns a depth-value to the pix el, thus creating a vox el w ithin the stack of slices.


as pixels) accumulate charges corresponding to the amount of light passing through the lens and onto each cell.1 As more light falls onto a photosite, the charge grows. Light is shut off to the lens once the shutter closes, at


which point the charge in each cell is recorded by a processing chip and converted to a digital value that determines the color and intensity of individual pixels used to dis- play the image on screen. Resolution in these


1 ,0 0 0 V ox e l


Color bar


2 5 6 1 ,0 0 0 8 0 0


6


Oilfield Review


V ertical coordinates, y


V ertical coordinates, y


Slice number


, z


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