Attributes of Graphics Primitives Chapter 4

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

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 ( );

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);

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

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.

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

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

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.

Example

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

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

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();

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

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

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

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

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

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.

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

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

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

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

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

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.

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.