Raster graphics

Raster graphics
Name	Raster graphics
Caption	A close-up view of a raster image showing individual pixels.
Extension	.jpg, .png, .gif, .bmp, .tiff
Mime	image/jpeg, image/png, image/gif, image/bmp, image/tiff
Developer	Various
Genre	Digital image

Raster graphics. A raster graphic, also known as a bitmap, is a digital image composed of a rectangular grid of pixels, or picture elements. Each pixel contains color information, and the collective arrangement of these pixels forms the complete image. This structure makes them exceptionally well-suited for representing complex, continuous-tone imagery such as photographs and detailed artwork. The quality and file size of a raster image are directly tied to its total number of pixels, known as its resolution.

Definition and basic principles

A raster image is fundamentally defined by a fixed grid of individual picture elements. Each pixel is assigned specific color values, typically defined in color models such as RGB for screen display or CMYK for printing. The dimensions of this grid, expressed as width and height in pixels, constitute the image's native resolution. When such an image is enlarged beyond this original resolution, individual pixels become visible, causing a blocky appearance known as pixelation. The amount of color data stored per pixel, called bit depth, determines the range of colors or shades of gray the image can represent, directly impacting its visual fidelity and file size.

Comparison with vector graphics

The primary alternative to raster graphics is vector graphics, which uses mathematical formulas to define lines, curves, and shapes. While raster images are resolution-dependent, vector graphics are resolution-independent and can be scaled to any size without loss of quality. This makes vector formats like SVG ideal for logos, typography, and technical drawings. Raster graphics, by contrast, are superior for depicting complex color gradients and realistic scenes, as in digital photography or painting. Software such as Adobe Photoshop is designed for raster manipulation, whereas Adobe Illustrator and Inkscape are standard tools for vector creation.

Common file formats

Numerous file formats exist for storing raster image data, each with distinct characteristics for compression and use. Lossy compression formats, such as JPEG, are ubiquitous for photographs on the web due to significantly reduced file sizes, though with some permanent data loss. Lossless compression formats, including PNG and GIF, preserve all original data; PNG supports transparency and is common for web graphics, while GIF is known for simple animations. Uncompressed or minimally compressed formats like BMP and TIFF are often used in professional publishing and medical imaging where data integrity is paramount.

Technical details and representation

The technical representation of a raster image involves a two-dimensional array of pixel values stored in computer memory. The raster scan order, typically from left to right and top to bottom, dictates how this array is read and displayed. Color information is often stored in channels, such as the red, green, and blue channels of the RGB color model. Advanced techniques like alpha channels enable transparency effects. The relationship between pixel dimensions and physical print size is governed by the pixels-per-inch (PPI) setting. High-resolution devices, such as those used in CT scans or modern smartphones, capture and display images with extremely high pixel densities.

Applications and uses

Raster graphics are the foundational technology for most digital visual media. They are the standard format for all digital camera output, from consumer DSLRs to images captured by the Hubble Space Telescope. The entire discipline of digital painting and photo editing relies on raster-based software. In entertainment, raster images form the textures and frames for computer-generated imagery in films from studios like Pixar and Disney. They are essential for broadcast television, video games, and the display of content on every major operating system and Web browser.

History and development

The origins of raster graphics are intertwined with the development of early computer displays and facsimile technology. A pivotal moment was the invention of the scanning drum for image digitization. The concept was commercially realized with systems like the LyonLamb VAS for video animation. The Adobe PostScript page description language, while primarily for vector graphics, helped standardize the handling of raster images in the Desktop publishing revolution. The explosive growth of the Internet and the creation of the W3C standardized formats like JPEG and PNG, cementing raster graphics as the universal language of digital pictures. Modern advancements continue in fields like digital cinematography and ultrasound imaging.

Category:Computer graphics Category:Digital photography Category:File formats