Attributes of Output Primitives Computer Graphhics.pptx
1. Attributes of Output Primitives
Structure :
Definition
Line Attribute
Curve Attribute
Color and Grayscale Level
Area Filled Attribute
Text and Characters
2. Attributes
Influence the way a primitive is displayed
Two main ways of introducing attributes:
1) added to primitive’s parameter list
e.g. setPixel(x, y, color)
2) a list of current attributes (to be updated
when changed)
e.g setColor(color); setPixel(x, y);
3. The way a primitive is to be displayed is referred to as an
Attribute Parameter.
Some attribute parameters include color ,size etc.
Different ways to incorporate attribute changes :
Extend the parameter list associated with each primitive
Maintain a system list of current attribute values and use
separate functions to set attributes.
Introduction
5. Attributes for lines
Lines (and curves) are normally infinitely thin
• type
– dashed, dotted, solid, dot-dashed, …
– pixel mask, e.g. 11100110011
• width
– problem with the joins; line caps are used to adjust shape
• pen/brush shape
• color (intensity)
6. Type :
Solid
Dotted – very short dash with spacing equal to or greater than dash itself
Dashed – displayed by generating an inter dash spacing
Dash Dotted –combination of the earlier two
To set line type attribute in PHIGS Application,
setLineType(lt) can be used. Lt can be -1,2,3,4
Attributes of Line
7. Raster Line algorithms display line attributes by plotting pixel spans.
Pixel count for the span and inter span length and inter span spacing can be
specified using the mask .
Ex. 111100011110001111
Plotting dashes with fixed number of pixels result in unequal-length dashes for
different line orientations. Horizontal line looks small when compared to a vertical
line.
For dash lines to remain constant, we should adjust the pix span el count for the
solid span and inter space span according to the slope of the line.
Attributes of Line
8. Line Width
Specify in pixels and proportion of a standard line width.
Thicker line can be produced by.
. Adding extra pixel vertically when |m| < 1
. Adding extra pixel horizontally when |m| > 1
We can set the width of a line using setLineWidthScaleFactor(lw);
Where lw is assigned a positive number.
9. Line Width
Issues:
Line have different thickness on the slope
Problem with
. End of the line (Use Line Caps)
. Joining the two lines (polygon)
12. Pen and Brush Options
The selected “pen” or “brush” determine the way a line will be drawn.
Pens and brushes have size, shape, color and pattern attribute.
Pixel mask is applied in both of them.
13. Line Color
A polyline procedure displays a line in current color by setting this
color value in the frame buffer at pixel locations along the line path
using the set Pixel function.
setPolyLineColorIndex(lc)
A line drawn with background color in invisible.
14. Curve Attributes
Similar to line : type + width
Thicker curves can be produced by:
1. Plotting additional pixel
2. Filling the space between two concentric circles.
3. Using thicker pen or brush
17. Color as attribute
Each color has a numerical value, or intensity, based on
some color model.
A color model typically consists of three primary colors,
in the case of displays Red, Green and Blue (RGB)
For each primary color an intensity can be given, either
0-255 (integer) or 0-1 (float) yielding the final color
256 different levels of each primary color means
3x8=24 bits of information to store
18. Color representations
Two different ways of storing a color value:
1) a direct color value storage/pixel
2) indirectly via a color look-up table
index/pixel (typically 256 or 512 different
colors in the table)
19. Color and GrayScale Levels
Color
General Purpose raster scan systems provide a variety of colors
while random scan monitors provide very few.
Colors are represented by colors codes which are positive integers.
Color information is stored in frame buffer or in separate table and
use pixel values as index to the color table.
Two ways to store color information :
1. Direct
2. Indirect
22. Gray Scale
Apply for monitor that have no color
Shades of grey (white->light grey->dark grey->black)
Color code mapped onto grayscale codes
2 bits can give 4 level of grayscale
8 bits per pixel will allow 256 combination
Dividing the actual code with 256 will give range of 0 and 1
Ex:
Color code in color display is 118
To map to nearest grayscale then
118/256 = 0.45
light gray
23. Area Fill Attributes
Option for filling a defined region is whether solid , pattern and colors.
Fill Styles
Three basic fill styles are:
1. Hollow with color border
2. Solid
3. Patterned
25. Area Fill Attributes
A basic fill style can be assigned in a PHIGS program using the following
function:
setInteriorStyle(fs),
Where fs c an be hollow,s olid,or pattern .
Another value for Fill Style is Hatch, which is used to fill an area with
selected hatching patterns.2 types :
Diagonal Hatch Fill and Diagonal Cross Hatch Fill.
26. Attributes for area fill
• fill style
– hollow, solid, pattern, hatch fill, …
• color
• pattern
– tiling
28. Area Fill Attributes
The color for a solid interior or for a hollow area outline is chosen with :
setInteriorColorIndex(fc) where fc is the desired color code.
Other fill options include specifications for the edge type, edge width edge
color of a region.
29. Pattern Fill
We select fill patterns with
setInteriorStyleIndex(pi), where pi specifies a table position
For fill style pattern, tables entries can be created on individual output
devices with :
setPatternRepresentation (ws,pi,nx,ny,cp)
Where,
pi is pattern index number,
ws is the workstation code,
Cp is the 2d array of color codes with nx columns and ny rows.
35. Antialiasing
Aliasing ≈ the fact that exact points are
approximated by fixed pixel positions
Antialiasing = a technique that compensates for
this (more than one intensity level/pixel is
required)
36. Antialiasing, a method
A polygon will be studied (as an example).
Area sampling (prefiltering): a pixel that is only partly
included in the exact polygon, will be given an
intensity that is proportional to the extent of the
pixel area that is covered by the true polygon
37. Area sampling
P = polygon intensity
B = background intensity
f = the extent of the pixel
area covered by the true
polygon
pixel intensity =
P*f + B*(1 - f)
Note! Time consuming to
calculate f
