1. Attributes of Graphics Primitives Chapter 4

2. Atrributes of Output Primitves
In general, any parameter that affects the way a
primitive displayed is an attribute parameter.
Ex: Line attributes
type (solid, dashed, dotted), width, cap (butt,
round, projecting square caps), join (miter,
round, etc.), color, grayscale

3. Point Attributes -OpenGL
glPointSize(GLfloat size);
Example : glColor3f ( 1.0, 0.0, 0.0);
glBegin(GL_POINTS);
glVertex2i(50,100);
glPointSize(2.0);
glColor3f ( 0.0, 1.0, 0.0);
glVertex2i(75,150);
glPointSize(3.0);
glColor3f ( 0.0, 0.0, 1.0);
glVertex2i(100,200);
glEnd ( );

4. 3 Line Attributes- OpenGL
Line Color – glcolor( );
Line Width - glLineWidth(GLfloat width);
Line Style - glLineStipple(repeatFactor, pattern);
Pattern – 16-bit integer ( 0xFFFF with each bit position 1 )
default is a solid line
Repeat factor specifies how many times each bit
in the pattern is repeated before the next bit in
the pattern is applied.- Default value is 1
Example:
glEnable(GL_LINE_STIPPLE);
glLineStipple(1, 0x00FF); // Plot dashed double width line
glLineWidth(2.0);
lineplot( ) ; // plot the line
glLineStipple(1, 0x0101); \\Plot dotted triple width line
glLineWidth(3.0);
lineplot( );
glDisable(GL_LINE_STIPPLE);

5. Polygons Filling Polygons Scan-line fill algorithm
Inside-Outside tests
Boundary fill algorithm 11 1 2 3 4 5 6 7 8 9 10 5 6 7 8 9 4 3 1 2

6. SCAN-CONVERTING POLYGONS
Find the intersections of the scan-line with all edges
of the polygon.
Sort the intersections by increasing the x
coordinate.
Fill in all pixels between pairs of intersections that
lie interior to the polygon, using the odd-parity rule
to determine that a point is inside a region. 6
.6.

7. Scan-Line Polygon Fill
Topological Difference between 2 Scan lines
y : intersection edges are opposite sides
y’ : intersection edges are same side y 1 2 2 y’ 1 1

8. Scan-Line Polygon Fill (cont.)
Edge Sorted Table B yC yB xC
1/mCB C C’ E yD
yC’ xD
1/mDC yE xD
1/mDE D yA yE xA
1/mAE yB xA
1/mAB A 1
Scan-Line Number

9. Procedure
Set y to the smallest y that has an entry in the ET(Edge Table – all edges sorted by smaller y cord
Initialize the AET(Active ET corresponds to current scan line) to be empty.
Repeat until the AET and ET are empty:
Move from ET bucket y to AET those edges whose ymin = y.
Sort the AET on x.
Fill-in desired pixel values on scan-line y by using pairs of x coord. from the AET.
Increment y by 1.
Remove from the AET those entries that y=ymax.
For each non-vertical edge remaining in the AET, update x for the new y: x = x + 1/m.

10. Example

11. Inside-Outside Tests
Self-Intersections – scan line polygon Fill Algorithm
Odd (Interior)-Even(Exterior) rule
exterior
interior

12. Boundary-Fill Algorithm
Proceed to Neighboring Pixels
4-Connected
8-Connected

13. OpenGL Implementation
A convex polygon means that all the angles inside the polygon formed by the edges are smaller than 180 degrees.
If a polygon is not convex, it is concave. Convex polygons can be scan-converted faster than concave polygons.
OpenGL draws a convex polygon with the following commands:
glBegin(GL_POLYGON);
// a list of vertices
glEnd();

14. Effects of Scan conversion
Aliasing : The distortion that result from scan conversion.
Staircase Effect ( jagged appearance)
Unequal Brightness( slanted line appears dimmer than a horizontal/vertical)
Picket Fence Problem(object is not aligned to the pixel grid)
Hence we go for Anti Aliasing

15. Antialiasing Aliasing Undersampling: Low-frequency sampling
Nyquist sampling frequency:
Nyquist sampling interval:
original
sample
reconstruct

16. Antialiasing Supersampling (Postfiltering)
Increase the sampling rate by
Sampling object charactersitics at a high resolution and display the results in low resolution is called
Area Sampling (Prefiltering)
Pixel Phasing
Shift the display location of pixel areas

17. SPECIFYING A COLOR -OpenGL
glColor3f(0.0, 0.0, 0.0);
draw_object(A);
draw_object(B);
glColor3f(1.0, 0.0, 0.0);
draw_object(C);
glColor3f(0.0, 0.0, 0.0); black
glColor3f(1.0, 0.0, 0.0); red
glColor3f(0.0, 1.0, 0.0); green
glColor3f(1.0, 1.0, 0.0); yellow
glColor3f(0.0, 0.0, 1.0); blue
glColor3f(1.0, 0.0, 1.0); magenta
glColor3f(0.0, 1.0, 1.0); cyan
glColor3f(1.0, 1.0, 1.0); white

18. Supersampling (postfiltering)
Increase sampling rate, each pixel area is
covered by a finer grid
We can then use multiple sample points in a pixel
that is within a line to determine an appropriate
intensity level for the pixel.
Supersampling algorithms often choose more weight to
subpixels near the center of a pixel area: pixel-
wighting masks

19. Area sampling (prefiltering)
Determine pixel intensity by calculating the areas of overlap of each pixel with the objects to be displayed
Unweighted Area Sampling
determines the pixel intensity by the overlap area only
Weighted Area Sampling
an area closer to the pixel’s center has greater influence on the pixel’s intensity than an equal area further away from the pixel’s center.

20. Supersampling Subpixels Increase resolution 22
(10, 20): Maximum Intensity
(11, 21): Next Highest Intensity
(11, 20): Lowest Intensity 21 20 10 11 12

21. Supersampling Subpixels (10, 20): Maximum Intensity –Level3
(11, 21), (12,21): Next Highest Intensity-2
(11, 20),(12,22): Lowest Intensity-1
Subpixels
Increase resolution 22 21 20 10 11 12

22. Pixel-Weighting Masks
Give More Weight to Subpixels Near the Center of a Pixel Area
Sum of the weights in the filter =1
Low pass Filtering 1 2 4
1/16
1/8
1/4

23. Area Sampling
Set Each Pixel Intensity Proportional to the Area of Overlap of Pixel
2 adjacent vertical (or horizontal) screen grid lines  trapezoid 22
(10, 20): 90%
(10, 21): 15% 21 20 10 11 12

24. Filtering Techniques Filter Functions (Weighting Surface) Box Filter
Cone Filter
Gaussian Filter

25. Character Generation Bitmapped Fonts:
Bitmapped Fonts:
 .Characters represented as a rectangular grid or bits.
.Simple and quick to draw because character is mapped
onto pixel positions in the frame buffer.
.However, it needs a lot of space. Each variation in size
and style needs to be stored separately in a font cache.
.Can still generate different size and styles from a
single bitmap but the results are poor quality because
bitmap descriptions do not scale well.

26. Character Generation .
Overall design or style for set of characters is called
type-face.
.Ex: Courier, Arial, Times New Roman.
.Type-face are serif and san-serif (optima).
.Font is originally referred to size and format.