Bitmapped and Vector graphics

In bitmapped graphics, an image is divides into a grid of pictures elements called pixels

Bitmapped Graphics

The binary codes for each pixel arestored in memory (before being written to a file) when an image is scanned.

An image is displayed on avisual display unit by firsttransferring the binary codesfrom a file back into memory

The term bitmap arised becuase the pixels of the image are mapped to specific positions in memeoery where binary codes - one per pixel -are stored

These codes represent the colour or grey-scale value of each pixel for images make up of black, white and shades of grey.

Resolution

Resolution

The screen of a VDU (Visual display unit) is divided intopixels. The higher the resolution, the greater the numberof pixels for a given size of screen.

An image is created on the screen by selectivly lighting the pixels in the required colours. The colour selected for each pixel is coded in a fixed number of bits(For example, 8 bits per pixel gives 256 possible colours.)

Resolution of an image : a quantity expressed as the number of dots/pixelsper inch or centimeter

Resolution of a VDU screen : a quanitityexpressed as the number of pixels perrow by the number of pixels per column.

It is not the number of pixels that detmines the quaility of an image but the size of each pixel, the smaller the better

Colour

Colour DepthThe number of bits used to represent the colour orgreyscale vale of a single pixel in a bitmappedimage is called the colour depthThe colour of a single pixel can be coded using theRGB colour model, which mixes R, G & B to produce a specific colour. In this model, the relativebrightness of R, G & B are encodede seperately.

1-bit ColourA black and whtie image is known as a monochrome image. One bit is allocted to each pixel, restricting the colour of each pixel to black or white. 0 is black, 1 is white.

12-bit ColourIn 12-bit direct colour, 4 bits (2^4 or 16 possible levels) are alloced for each of the R, G & B components, enabling 4,096 (16 x16 x16) different colours. this colour depth is sometimes used in deviced with a limited colour display, such as moblies.

True ColourTrue colour can mimic many colours found in the real world, producing 16.7 million distinct colours. This apporeached the level at which the human eye can distingush colours for most photographic images.  24-bit true colour uses 8 bits to represent red, 8 bits to represent blue and 8 bits to represent green, giving 2^8 (564) levels of each of the three colour components. They can therefore be combined to give a total of 16,777,216 (256 x 256 x 256) colours.

32-bit Colour32-bit colour is really true colour with an extra 8 bits chosen beacuse the wrod size of a modern PC's is 32 or 64 bits. The extra 8 bits are either ignored or used to represent an alpha channel, which is a way of providing partial transparency.

Memory Requirements

If an image is 8 x 8 x 8 (64 pixels in total), a minimum of 64 bytes is required to store the images bitmap. If 26 bits are used per pixel then a minium of 128 bytes are needed for the bitmap. Using 32 bits per pixel required 256 bytes.

If a higher resolution is used for an image, keeping its dimensions the same, we need to create a bitmap of more pixels.For example is we double the size in pixels in each dimensions from 8x 8 to 16 x 16, we increate the number of pixels by a factor of 4, so our minuim memory requirment must also increace by a factor of 4. Using 32 bits per pixel will require a minium of 1,024 bytes.

Bitmap file calculations

Increasing the resolution of a bitmapped VDU screen increased the amount of memory required to display graphics, as does increasing the colour depth. For example, a screen set to a resolution of 1024 x 768 and a colour depth of 32 bits needs a memory of 1024 x 768 x 4 bytes = 3,145,728 bytes = 3 MBThere at least 3MB of space is required to save the displayed graphic in a file. Actually, the file space required will be slightly greater beacuse the resolution and colour depth information must be saved at the beginning of the file in an area called the header.

Questions1) What is a pixel?2) What is a bitmap?3) What is meant by the resolution of an image?4) what is menat by colour depth?5) Calculate the minumum file size of a 1024 x 768 pixels bitmapped image that used a colour depth of 24 bits.

Vector Graphics

A vector graphic identifies the objects that make up images - the lines, arhcs, curves, circles ect. Even text is broken down into straight lines and curves.

Vector graphics record information about these objects to define the image. Pixels do not enter into this process.

For exmaple, a line 100 pixels long needs jsut a few bytes to describe it in a vector graphic, whereas 100 x 3 bytes are required in a bitmap using 24-bit colour.

Vector drawing software uses the commands that create objects as the source of infomation needed to describe the graphic

If you are converting a bitmapped graphic to a vector graphic, the software uses techniques that identify the graphic components of the bitmap.

Properties of objectsTo recreate a vector graphic, the propertires of every object in the drawing list must be specified. Properties or attributes are position, size, direction (possible using endpoitn co-ordinantes) line thickness, font size and typeface, shading, the mathmatical description of the curves, whether a closed shape is filled or not, the fill colour, whether the closed shape has a border, the colour and thickness of any border.

Drawing List - Object infomation (in vector graphic format) is recordered in a file. This format lists all the objects making up the graphic with nessesery infomation about each pbject as a list of drawing commands.

Comparing Bitmapped and Vector graphics

When a bitmapped is scaled, the pixels enlarge. As the magnification increases, the pixels can become visible, leading to a 'staircase' effect.

Vector graphics avoid such distortion when magnified because caling is applied to a line thickness in a way that avoids creating such an effect. The scaling is geometric because vector graphics deal with objects not pixels. Vector graphics are defined by the relative positions of objects in a three dimentional or two dimensional coordinate system. If you increase the size of the coordinate system, the drawing increase sin size. You do not lose resolution when you enlarge a true vector drawing, becuase it has no intrinsic resolution.

Geometric images require fewer bytes in vector graphic format than in bitmapped format

Images that have continuois areas of colour, such a photopgraphic images, take up fewer bytes in bitmapped format than in vector format

Geometric images load faster from secondary storage (magnetic disk) and download faster over the internet in vector graphic format

Bitmap

Vector

Vector graphic images scale without distortino whereas bitmapped images do not

Some vector graphic formats can be searched for particular graphic objects which may then be manipulated, it is much harder to do this in bitmapped graphics

Basic Compression Techniques

Run-length encodingRLE  is a simple compression technique that t

Lossy Compression

The purpose of data compression is to squeeze data into a smaller number  of bytes than the data would occupy is uncompressed. Bitmapped graphic files can be very large, e.g 3 MB for a resolution of 1024 x 768 and a colour depth of 32 bit, but it is possible to reduce the size of bitmapped files by variour compresion techniques.

Bitmap

Vector

Comparing Bitmapped and Vector