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Photographs, magazines and other objects of nature such as an apple; create color by subtracting or absorbing certain wavelengths of color while reflecting other wavelengths back to the viewer. This phenomenon is called subtractive color.

A red apple is a good example of subtractive color; the apple really has no color; it has no light energy of its own, it merely reflects the wavelengths of white light that cause us to see red and absorbs most of the other wavelengths which evokes the sensation of red. The viewer (or detector) can be the human eye, film in a camera or a light-sensing instrument.

The subtractive color system involves colorants and reflected light. Subtractive color starts with an object (often a substrate such as paper or canvas) that reflects light and uses colorants (such as pigments or dyes) to subtract portions of the white light illuminating an object to produce other colors. If an object reflects all the white light back to the viewer, it appears white. If an object absorbs (subtracts) all the light illuminating it, no light is reflected back to the viewer and it appears black. It is the subtractive process that allows everyday objects around us to show color.

Color paintings, color photography and all color printing processes use the subtractive process to reproduce color. In these cases, the reflective substrate is canvas (paintings) or paper (photographs, prints), which is usually white.

What is Color?

Color is all around us. It is a sensation that adds excitement and emotion to our lives. Everything from the cloths we wear, to the pictures we paint revolves around color. Without color; the world (especially RGB World) would be a much less beautiful place. Color can also be used to describe emotions; we can be red hot, feeling blue, or be green with envy.

In order to understand color we need a brief overview of light. Without light, there would be no color, and hence no RGB World. Thank God for light!

Light is made up of energy waves which are grouped together in what is called a spectrum. Light that appears white to us, such as light from the sun, is actually composed of many colors. The wavelengths of light are not colored, but produce the sensation of color.

Raster Images

• A Raster image is a collection of dots called pixels.
  
• Each pixel is a tiny colored square.
  
• When an image is scanned, the image is converted to a collection of pixels called a raster image
  
• Scanned graphics and web graphics (JPEG and GIF files) are the most common forms of raster images.
  
• The quality of an imprint produced from a raster image is dependant upon the resolution (dpi) of the raster image, the capabilities of the printing technology and whether or not the image has been scaled up.
  

Vector Images

• A vector image is a collection of connected lines and curves that produce objects.
  
• When creating a vector image in a vector illustration program, node or drawing points are inserted and lines and curves connect notes together.
  
• Each node, line and curve is defined in the drawing by the graphics software by a mathematical description.
  
• Text objects are created by connecting nodes, lines and curves.
  
• In a vector object, colors are like clothes over the top of a skeleton.
  
• They can be scaled up or down without any loss of quality.
  
• Since vector images are composed of objects not pixels, you can change the color of individual objects without worrying about individual pixels.
  

Scanline rendering is the preferred method for generating most computer graphics in motion pictures. One particular implementation, REYES, is so popular that it has become almost standard in that industry. Scanline rendering is also the method used by video games and most scientific/engineering visualization software (usually via OpenGL). Scanline algorithms have also been widely and cheaply implemented in hardware.

In scanline rendering, drawing is accomplished by iterating through component parts of scene geometry primitives. If the number of output pixels remains constant, render time tends to increase in linear proportion to the number of primitives. OpenGL and Photorealistic Renderman are two examples of scanline rendering.

Before drawing, a Z or depth buffer containing as many pixels as the output buffer is allocated and initialized. The Z buffer is like a heightfield facing the camera, and it keeps track of which scene geometry part is closest to the camera, making hidden surface removal easy. The Z buffer may store additional per-pixel attributes, or other buffers can be allocated to do this (more on this below). Unless primitives are prearranged in back-to-front painting order and do not present pathological depth issues, a Z buffer is mandatory.

For each primitive, it is either composed of an easily drawable part (usually a triangle) or can be divvied up (tesselated) into such parts. Triangles or polygons that fit within screen pixels are called micropolygons, and represent the smallest size a polygon needs to be for drawing. It is sometimes desriable (but not absolutely necessary) for polygons to be micropolygons -- what matters is how simply (and therefore quickly) a polygon can be drawn.

Assigning color to output pixels using these polygons is called rasterization. After figuring out which screen pixel locations the corners of a polygon occupy, the polygon is scan-converted into a series of horizontal or vertical strips (usually horizontal). As each scanline is stepped through pixel by pixel (from one edge of the polygon to the other), various attributes of the polygon are computed so that each pixel can be colored properly. These include surface normal, scene location, z-buffer depth, and polygon s,t coordinates. If the depth of a polygon pixel is nearer to the camera than the value for the respective screen pixel in the Z buffer, the Z buffer is updated and the pixel is colored. Otherwise, the polygon pixel is ignored and the next one is tried.