Color of (digital image)

Raed S. Rasheed2012

Agenda

• Color.• Color Image.• Color Models– RGB color model.– CMYK color model.– HSV and HSL color model.

• Channel (digital image) • Color Depth• Image Resolution

Color

• Color is the visual perceptual property corresponding in humans to the categories called red, yellow, white, etc.

Color Image

• color image is a digital image that includes color information for each pixel.

• For visually acceptable results, it is necessary (and almost sufficient) to provide three samples (color channels) for each pixel, which are interpreted as coordinates in some color space. The RGB color space is commonly used in computer displays, but other spaces such as YCbCr, HSV, and are often used in other contexts.

Color Models

• A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components (e.g. RGB and CMYK are color models).

RGB color model

• The RGB color model is an additive color model in which red, green, and blue light is added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three additive primary colors, red, green, and blue.

• The main purpose of the RGB color model is for the sensing, representation, and display of images in electronic systems, such as televisions and computers, though it has also been used in conventional photography.

CMYK color model

• The CMYK color model (process color, four color) is a subtractive color model, used in color printing, and is also used to describe the printing process itself. CMYK refers to the four inks used in some color printing: cyan, magenta, yellow, and key (black).

HSL and HSV color model

• HSL and HSV are a color models which has components for Hue (the color, such as blue or red), Saturation (how strong the color is) and Value (the brightness).

HSV color model• Hue

– This is the color itself, which results from the combination of primary colors. All shades (except for the gray levels) are represented in a chromatic circle: yellow, blue, and also purple, orange, etc. The chromatic circle (or “color wheel”) values range between 0° and 360°. (The term “color” is often used instead of “Hue”. The RGB colors are “primary colors”.)

• Saturation– This value describes how pale the color is. A completely unsaturated color is a shade of

gray. As the saturation increases, the color becomes a pastel shade. A completely saturated color is pure. Saturation values go from 0 to 100, from white to the purest color.

• Value– This value describes the luminosity, the luminous intensity. It is the amount of light emitted

by a color. You can see a change of luminosity when a colored object is moved from being in the shadow to being in the sun, or when you increase the luminosity of your screen. Values go from 0 to 100. Pixel values in the three channels are also luminosities: “Value” in the HSV color model is the maximum of these elementary values in the RGB space (scaled to 0-100).

Channel (digital image)

• A channel in this context is the grayscale image of the same size as a color image, made of just one of these primary colors. For instance, an image from a standard digital camera will have a red, green and blue channel. A grayscale image has just one channel.

Channel (digital image)

• RGB three• CMYK four• HSV three

Channel (digital image)

• Alpha channel– The alpha channel stores transparency

information—the higher the value, the more opaque that pixel is. No camera or scanner measures transparency, although physical objects certainly can possess transparency, but the alpha channel is extremely useful for compositing digital images together.

Color Depth

• In digitizing images, the color channels are converted to numbers. Since images contain thousands of pixels, each with multiple channels, channels are usually encoded in as few bits as possible. Typical values are 8 bits per channel or 16 bits per channel.

Color Depth• 1-bit color (21 = 2 colors) monochrome, often black and white, compact

Macintoshes, Atari ST.• 2-bit color (22 = 4 colors) CGA, gray-scale early NeXTstation, color Macintoshes,

Atari ST.• 3-bit color (23 = 8 colors) many early home computers with TV displays• 4-bit color (24 = 16 colors) as used by EGA and by the least common

denominator VGA standard at higher resolution, color Macintoshes, Atari ST.• 5-bit color (25 = 32 colors) Original Amiga chipset• 6-bit color (26 = 64 colors) Original Amiga chipset• 8-bit color (28 = 256 colors) most early color Unix workstations, VGA at low

resolution, Super VGA, color Macintoshes, Atari TT, AGA, Falcon030.• 12-bit color (212 = 4096 colors) some Silicon Graphics systems, Neo Geo, Color

NeXTstation systems, and Amiga systems in HAM mode.• 16-bit color (216 = 65536 colors) some color Macintoshes.

Image Resolution

• Image resolution is an umbrella term that describes the detail an image holds. The term applies to raster digital images, film images, and other types of images. Higher resolution means more image detail.

Image Resolution• Analog and early digital

– 352×240 : Video CD– 300×480 : Umatic, Betamax, VHS, Video8– 350×480 : Super Betamax, Betacam– 420×480 : LaserDisc, Super VHS, Hi8– 640×480 : Analog broadcast (NTSC)– 670×480 : Enhanced Definition Betamax– 768×576 : Analog broadcast (PAL, SECAM)

• Digital– 720×480 : D-VHS, DVD, miniDV, Digital8, Digital Betacam– 720×480 : Widescreen DVD (anamorphic)– 1280×720 : D-VHS, HD DVD, Blu-ray, HDV (miniDV)– 1440×1080 : HDV (miniDV)– 1920×1080 : HDV (miniDV), AVCHD, HD DVD, Blu-ray, HDCAM SR– 2048×1080 : 2K Digital Cinema– 4096×2160 : 4K Digital Cinema– 7680×4320 : UHDTV– Sequences from newer films are scanned at 2,000, 4,000, or even 8,000 columns, called 2K, 4K, and 8K, for

quality visual-effects editing on computers.