1. o عَلَّمَهُ الْبَيَانَ
Allah says
o عَلَّمَهُ الْبَيَانَ
He has taught him speech (and intelligence).
Al-Qur'an, (Ar-Rahman)

2. Majed Bouchahma boumaged@gmail.com
Lecture-3 Attributes of Graphics Primitives : Attributes of Line Primitives
Majed Bouchahma

3. Introduction
In general, a parameter that affects the way a primitive is to be displayed is referred to as an attribute parameter i.e: size, color,…
OpenGL assign attributes setting using separate functions that update a system attribute list.
A graphic system that maintain a list for the current values of attributes is referred to as a state system.
Attributes of output primitives are referred to as state variables.

4. OpenGL color functions
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB)
1st parameter: Frame Buffer
2nd parameter: Color mode
In RGB mode we select the current color components using the function:
glColor*(Colorcomponents)
i.e:glColor3i(0,255,255)

5. Line Attributes Line Colour Line Type Solid Lines Dashed Lines
Set by changing the pixel colour while drawing
Line Type
Different types of lines can be drawn by setting the length and spacing of displayed solid sections along the line path
Solid Lines
Dashed Lines
Inter-dash spacing equal to the solid section
Dotted Lines
Inter-dash spacing greater than the solid section

6. Line Attributes… Line Type… (See Book P.185) Use of pixel masks
Pixel counts along the line path for the solid and inter-line spacing can be specified by using pixel masks
Bit strings containing ones and zeros
Draw 4 pixels and then give spacing of 3 pixels

7. Line Attributes… Line Width ( See book P:183)
Thicker lines can be displayed as positive integer multiples of the standard line by plotting additional pixels along adjacent line paths

8. Brush and Pen Options
The pen or the brush is the name of the tool used for drawing a line
Its shape changes the way a line is drawn
Brush Options

9. Brush and Pen Options…
A brush can be represented by a pixel mask in the form of a matrix which stores the pixel values to be set at each pixel location along the path of the line 1 1

10. Curve Attributes
Can be set in a similar ways as line attributes

11. Anti-aliasing in Line Drawing
Displayed primitives generated by the raster algorithms contain jaggies
The sampling process digitizes coordinates points on an object to discrete integer pixel positions
This distortion of information due to under-sampling is called aliasing
Can be removed through anti-aliasing algorithms
There are no samples (pixels) available between the two steps which leads to under-sampling and as a result jaggies are produced

12. Anti-Aliasing through Super-Sampling
Divide each pixel into a number of sub pixels and count the number of sub pixels that are along the line path
Set the intensity level of each pixel in proportion to the sub pixel count

13. Anti-Aliasing through Super-Sampling
Example
Each pixel broken into 9 (3x3) sub pixels
Bresenham algorithm now selects which of the sub-pixels will be turned on during line drawing
Maximum number of pixels that can be turned on by the line drawing algorithm is 3
Therefore the number of color levels for each pixel is 3
The pixel colors are set proportional to the number of sub pixels turned on
Pixel level for a pixel with all 3 pixels on = line color
Pixel level for a pixel with no pixels on = background color

14. Anti-Aliasing through Super-Sampling
Example
Each pixel broken into 9 (3x3) sub pixels
Bresenham algorithm now selects which of the sub-pixels will be turned on during line drawing
Maximum number of pixels that can be turned on by the line drawing algorithm is 3
Therefore the number of color levels for each pixel is 3
The pixel colors are set proportional to the number of sub pixels turned on
Pixel level for a pixel with all 3 pixels on = line color
Pixel level for a pixel with no pixels on = background color
Line intensity is spread out over a greater number of pixels and the stairstep effect is smoothed by displaying a blurred line path in the vicinity of the stair step

15. End of Lecture-3
Work expands so as to fill the time available for its completion.
Cyril Northcote Parkinson (1909–1993)
British political scientist, historian, and writer.
Parkinson's Law (1958).