Image Processing Basics

Image Basics

> Pixels
> The term pixel is derived from "Picture Element". A pixel is a tiny dot of a particular color; images are made up of many of these dots arranged in rows and columns. Whether being displayed on your computer screen or printed on film or paper, digital images are made up the same rows and columns of pixels. By zooming an image in to a high magnification factor, you can see its individual pixels.
> How transparencies or prints are digitized:
> When a photograph is digitized, a scanner measures the color of the image at each pixel location and stores this information as three numbers representing the amounts of each of the three additive primary colors: red, green, and blue. Each color component is stored as 8 bits of data that gives a range of values from 0 to 255. A color pixel therefore consists of 24 bits of data; this means that each pixel can represent any of about 16 million colors.
> How digital images are displayed or printed
> To view a digital image, whether on a computer screen, as a transparency, or as a print, its digital data must be converted back to a physical color image. In the case of a display screen this is done by illuminating tiny clusters of red, green, and blue dots on the face of the picture tube, one for each pixel, in proportion to that pixel's red, green, and blue values. To create a transparency, a film recorder photographs an internal screen that exposes the film a line at a time. Digital printers use a wide variety of technologies from evaporating tiny dots of colored dyes onto special paper to exposing photographic paper directly from a laser or internal screen to create color images.
> Resolution
> The quality of a digital image is greatly affected by its resolution. The resolution of an image is the number of pixels per inch. Like film grain, the smaller the pixels, the sharper the image can be. High quality images are thus made up of a very large number of pixels.

> Color Images
> Color images are made up of color pixels, each of which is represented internally in the computer as three bytes (24 bits) defining its red, green, and blue values. This means that a color image 1000 pixels wide and 1000 pixels high occupies 3MB of memory.
> Black and White (Grayscale) Images
> Black and white images are made up of gray pixels each of which is represented internally as a single byte (8 bits) whose value describes its brightness. Thus a black and white image uses three times less memory than a color image of the same dimensions. You can use black and white images as selections or alpha channels for other images.

> Binary Images
> Binary images are made up of pixels that are either black or white; each binary pixel is represented as a single bit. Binary images use eight times less memory than black and white images.

> In Focus Photoeditor any image, acquired from any source, is automatically opened and edited as a Color Image at 24 bits per pixel. Layers are considered images with 32bits per pixel (24 + 8 alpha channel).

> Here is the meaning of some of the most common terms used in digital photography about color images:

> Brightness: is related to the intensity of the pixel.

> Saturation : refers to the intensity of a color. A fully saturated color is very pure and deep; as you reduce their saturation, colors become progressively more washed out until at a zero saturation they become shades of gray.

> Hue: is a measure of the wave length of the color: the wave length is associated to the visible spectrum of the light and divides the colors into: red, yellow, green, blue, etc..These are the colors we can see in the Rainbow, when the white light is splitted into several parts.

> Color System: A color system is a system of coordinates by which one can distinguish among the 16000000 and over colors that human eye can see. Some Color Systems (or Color Spaces) are RGB (Red-Green-Blue) used for displaying pictures on Monitors of Pc and TV, CMYK (Cyan-Magenta-Yellow-Black) used for printing pictures.

> HLS: this is the acronym for: Hue-Luminosity-Saturation color system. The complementary of HLS is HVS (Hue-Value-Saturation).

> Histogram: This is a map of the intensities of all the pixels in a given picture. To obtain an histogram we scan each pixel of the picture and read its intensity, then we report on a diagram the number of pixels in the picture that has an intensity of 0(min) , the number of pixels with intensity of 1 etc.. until intensity=255 (max). If we put on the vertical axis the Number of pixels and on the horizontal axis the possible intensity values between 0 and 255 we make an Histogram of the picture intensities.

> Generally we can adjust the histogram of a picture by stretching the histogram between a minimum and maximum: in the histogram above the correct values for min and max should be about 30 for Min and about 215 for Max: the reason for these values is that there are not so many pixels having a brightness less than 30 and more than 215. This operation is infact called "Histogram Stretch".

> Contrast: is not a characteristic of each single pixel but of the overall appearence of the picture: if the histogram of a picture is not well stretched over the full range of tones the picture lacks contrast.