Raster Graphics
Merci Malagarie
In computer graphics and digital photography, a raster graphics represents a two-dimensional picture as a rectangular matrix or grid of square pixels, viewable via a computer display, paper, or other display medium. A raster is technically characterized by the width and height of the image in pixels and by the number of bits per pixel.[citation needed] Raster images are stored in image files with varying dissemination, production, generation, and acquisition formats.
The word "raster" has its origins in the Latin rastrum (a rake), which is derived from radere (to scrape). It originates from the raster scan of cathode ray tube (CRT) video monitors, which paint the image line by line by magnetically or electrostatically steering a focused electron beam.[2] By association, it can also refer to a rectangular grid of pixels. The word rastrum is now used to refer to a device for drawing musical staff lines.
The fundamental strategy underlying the raster data model is the tessellation of a plane, into a two-dimensional array of squares, each called a cell or pixel (from "picture element"). In digital photography, the plane is the visual field as projected onto the image sensor; in computer art, the plane is a virtual canvas; in geographic information systems, the plane is a projection of the Earth's surface. The size of each square pixel, known as the resolution or support, is constant across the grid.Raster or gridded data may be the result of a gridding procedure.
Most computer images are stored in raster graphics formats or compressed variations, including GIF, JPEG, and PNG, which are popular on the World Wide Web.[3][4] A raster data structure is based on a (usually rectangular, square-based) tessellation of the 2D plane into cells, each containing a single value. To store the data in a file, the two-dimensional array must be serialized. The most common way to do this is a row-major format, in which the cells along the first (usually top) row are listed left to right, followed immediately by those of the second row, and so on.
Vector images (line work) can be rasterized (converted into pixels), and raster images vectorized (raster images converted into vector graphics), by software. In both cases some information is lost, although certain vectorization operations can recreate salient information, as in the case of optical character recognition.
Early mechanical televisions developed in the 1920s employed rasterization principles. Electronic television based on cathode-ray tube displays are raster scanned with horizontal rasters painted left to right, and the raster lines painted top to bottom.
Modern flat-panel displays such as LED monitors still use a raster approach. Each on-screen pixel directly corresponds to a small number of bits in memory.[5] The screen is refreshed simply by scanning through pixels and coloring them according to each set of bits. The refresh procedure, being speed critical, is often implemented by dedicated circuitry, often as a part of a graphics processing unit.
An early scanned display with raster computer graphics was invented in the late 1960s by A. Michael Noll at Bell Labs,[7] but its patent application filed February 5, 1970, was abandoned at the Supreme Court in 1977 over the issue of the patentability of computer software.[8]
During the 1970s and 1980s, pen plotters, using Vector graphics, were common for creating precise drawings, especially on large format paper. However, since then almost all printers create the printed image as a raster grid, including both laser and inkjet printers. When the source information is vector, rendering specifications and software such as PostScript are used to create the raster image.
Geographic phenomena are commonly represented in a raster format in GIS. The raster grid is georeferenced, so that each pixel (commonly called a cell in GIS because the "picture" part of "pixel" is not relevant) represents a square region of geographic space.[10] The value of each cell then represents some measurable (qualitative or quantitative) property of that region, typically conceptualized as a field. Examples of fields commonly represented in rasters include: temperature, population density, soil moisture, land cover, surface elevation, etc. Two sampling models are used to derive cell values from the field: in a lattice, the value is measured at the center point of each cell; in a grid, the value is a summary (usually a mean or mode) of the value over the entire cell.
Raster graphics are resolution dependent, meaning they cannot scale up to an arbitrary resolution without loss of apparent quality. This property contrasts with the capabilities of vector graphics, which easily scale up to the quality of the device rendering them. Raster graphics deal more practically than vector graphics with photographs and photo-realistic images, while vector graphics often serve better for typesetting or for graphic design. Modern computer-monitors typically display about 72 to 130 pixels per inch (PPI), and some modern consumer printers can resolve 2400 dots per inch (DPI) or more; determining the most appropriate image resolution for a given printer-resolution can pose difficulties, since printed output may have a greater level of detail than a viewer can discern on a monitor. Typically, a resolution of 150 to 300 PPI works well for 4-color process (CMYK) printing.
Raster-based image editors, such as PaintShop Pro, Corel Painter, Adobe Photoshop, Paint.NET, Microsoft Paint, and GIMP, revolve around editing pixels, unlike vector-based image editors, such as Xfig, CorelDRAW, Adobe Illustrator, or Inkscape, which revolve around editing lines and shapes (vectors). When an image is rendered in a raster-based image editor, the image is composed of millions of pixels. At its core, a raster image editor works by manipulating each individual pixel.[4] Most[citation needed] pixel-based image editors work using the RGB color model, but some also allow the use of other color models such as the CMYK color model.[12]
Since raster images are pixel based, they are resolution dependent. The number of pixels that make up an image as well as how many of those pixels are displayed per inch, both determine the quality of an image. As you may have guessed, the more pixels in the image and the higher the resolution is, the higher quality the image will be.
For example, if we scale a raster image to enlarge it, without changing resolution, it will lose quality and look blurry or pixilated. This is because we are stretching the pixels over a larger area, thus making them look less sharp. This is a common problem but can be remedied by using raster image editing programs such as Photoshop to change resolution and properly scale images.
Instead of trying to keep track of the millions of tiny pixels in a raster image, vector images keep track of points and the equations for the lines that connect them. Generally speaking, vector images are made up of paths or line art that can infinitely scalable because they work based on algorithms rather than pixels.
So, what types of graphics would typically be vector? Well, almost all computer font files are based on vector images of the letters - that's why it's possible to scale them WAY up or WAY down and still have the letters be clear. All Microsoft Office clipart uses vector art and most charts and graphs produced by Office or by statistical analysis software are vector-based. Typically vector art is created in illustration applications such as Adobe Illustrator or CorelDRAW. Vector illustrations are great for logos, illustrations/artwork, animations, and text.
Pixel: In Computer graphics, a pixel, dot, or picture element is a physical point in a picture. A pixel is simply the smallest addressable element of a picture represented on a screen. A majority of pictures that we see on our computer screen are raster images. The selfie that you click with your mobile phone is another example of a raster image. An image is made up using a collection of pixels referred to as a bitmap.
Bitmap: In computer graphics, a bitmap is a mapping from some domain (for example, a range of integers) to bits, that is, values which are zero or one. It is also called a bit array or bitmap index. The more general term pixmap refers to a map of pixels, where each one may store more than two colors, thus using more than one bit per pixel. Often bitmap is used for this as well. In some contexts, the term bitmap implies one bit per pixel, while pixmap is used for images with multiple bits per pixel.
Raster images use bit maps to store information. This means a large file needs a large bitmap. The larger the image, the more disk space the image file will take up. As an example, a 640 x 480 image requires information to be stored for 307,200 pixels, while a 3072 x 2048 image (from a 6.3 Megapixel digital camera) needs to store information for a whopping 6,291,456 pixels. We use algorithms that compress images to help reduce these file sizes. Image formats like jpeg and gif are common compressed image formats. Scaling down these images is easy but enlarging a bitmap makes it pixelated or simply blurred. Hence for images that need to scale to different sizes, we use vector graphics.
Making use of sequential commands or mathematical statements or programs which place lines or shapes in a 2-D or 3-D environment is referred to as Vector Graphics. Vector graphics are best for printing since it is composed of a series of mathematical curves. As a result vector graphics print crisply even when they are enlarged. In physics: A vector is something that has a magnitude and direction. In vector graphics, the file is created and saved as a sequence of vector statements. Rather than having a bit in the file for each bit of line drawing, we use commands which describe a series of points to be connected. As a result, a much smaller file is obtained.
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