topic 5 attributes of graphics primitives cgmb214: introduction to computer graphics
TRANSCRIPT
TOPIC 5ATTRIBUTES OF GRAPHICS PRIMITIVES
CGMB214: Introduction to Computer Graphics
Objectives
To understand the characteristic of a primitive
Color and Gray Scale
Color and Gray Scale
Color – A basic attribute for all primitivesVarious color option depend on capabilities
and design objectives of a particular system
RGB Color Components
In color raster system (monitor) Number of color choices available depends on the
amount of storage provided per pixel in the frame buffer
Two method of storing color information in the frame buffer Directly – storing RGB color code directly in the
frame buffer Indirectly – put the color codes into a separate
table and use the pixel locations to store index values referencing the color-table entries
RGB Color Components
Color Code
Stored Color Values in Frame Buffer Displayed Color
RED GREEN BLUE
0 0 0 0 Black
1 0 0 1 Blue
2 0 1 0 Green
3 0 1 1 Cyan
4 1 0 0 Red
5 1 0 1 Magenta
6 1 1 0 Yellow
7 1 1 1 White
RGB Color Components
For 24 bit color Store 8 bits each of red,
green and blue perpixel E.g (255,0,0) is pure red
and (255,255,255) is white
Yields 224 = 16 mill colorsFor 15 bit colors
5 bit red+5 bit green+5 bit blue
The video hardware uses the value to drive the R,G and B guns
255 0 0
RGB Color Components
Disadvantage As more color options are provided, the storage
required for the frame buffer also increases 1024 x 1024 full color (24-bit per pixel) RGB system
needs 3MB of storage for the frame buffer
Solution Storing the color values in a color lookup table (color
map)
Color Tables
Color Tables
Values stored in the frame buffer are now used as indices into the color table
Advantages Reduce the frame-buffer storage requirement to
1MB Table entries can be change at any time, allowing a
user to be able to experiment easily with different color combinations without changing the attribute setting for the graphics data structure
When a color value in the color table is changed, all pixels with that color index will change to the new color Without color table, we can only change the color of a pixel
by storing the new color at that frame buffer location
Point
Point Attributes
Two attributes of point Color
Set with RGB values or an index into a color table
Size An integer multiple of the pixel size Large point is displayed as a square
block of pixels
Line
Line Attributes
Basic attributes Color
Set with the same function for all graphics primitives (using RGB values)
Width Define with separate line functions
Style Define with separate line functions
Special effects for lines Pen Brush Strokes
Line Width
A standard width line is generated with single pixels at each sample position
Thicker lines are displayed as positive integer multiplies of the standard line Methods
If line slope magnitude ≤ 1.0 use vertical span Plotting a vertical span of pixels in each column (x position)
along the line. If line slope magnitude > 1.0 use horizontal span
Plotting a pixels to the right and left of the line path.
Line Width
Advantage vertical/horizontal span pixel Quick to generate the thick line
Disadvantage The displayed width of a line is dependent to its
slope i.e. 45 degree line will displayed thinner
compare to vertical and horizontal line The line ends are horizontal or vertical regardless
of the slope of the lineSolution
Adjust the shape of the line ends to give them a better appearance
Shape of the Line Ends
Square ends that are perpendicular to the line path
Line ends are added with a filled semi circle. Centered at the middle of the thick line. Diameter = the line thickness
Extend the line and add butt caps that are positioned ½ of the line width
Line Width
Other approach producing a thick line Displaying the line as a filled rectangle Generating the line with a selected pen or brush
Smooth Connection Line Segment
Method discussed will not produce a smoothly connected series of line segments
It leaves pixels gaps at boundary between line segments with different slope where there is a shift from horizontal pixel spans to vertical pixel spans
Additional processing at the segment endpoints is required to solve the problem
Smooth Connection Line Segment
3 possible methods for smoothly joining 2 line segments
Extending the outer boundaries of each of the two line segments until they meet
Capping the connecting between two segments with a circular boundary whose diameter = line width
Displaying the line segments with butt caps and filling in the triangular gap where the segments meet
Line Style
Type Solid Dotted – very short dash with spacing equal to or greater
than dash itself Dashed – displayed by generating an interdash spacing
Pixel count for the span and interspan length is specified by the mask E.g. 111100011110001111
Note: fixed pixel with dashes can produce unequal length dashes. It depends on line orientation. So need to adjust the number of plotted pixel for different slopes
Pen and Brush
The selected “pen” or “brush” determine the way a line will be drawn
Pens and brushes attributes are size, shape, color and pattern
Pixel mask is applied in both of them
Curve Attributes
Similar to line: type + width
Thicker curve can be produced by Plotting additional
pixel Filling the space
between two concentric circles
Using thicker pen or brush
Character
Character Attributes
We control the appearance of displayed character with attributes such as Font size Color Orientation
Character Attributes
Text-display option Typeface – choice of font
Set of character with a particular design i.e. courier, helvetica, times roman etc.
Assorted underlining styles Solid, dotted, double
Boldface Italic Emboss Shadow style
Character Attributes
Color settings Can be stored in the system attribute list
and used by the procedures that generate character definition in the frame buffer
Current color is used to set pixel values in the frame buffer corresponding to the character shapes and position
Character Attributes
Text size By scaling the overall dimensions of
character Height Width
Or scaling the height only Or scaling the width only
Character Attributes
Orientation Set according to the direction of a character up vector
Orientation of character from baseline to capline is in the direction of up vector
Set using text-path orientation Arrange the character strings vertically or horizontally or
forward or backward Adjust the position of the individual characters in the
frame buffer according to the option selected Combination of up-vector and text-path to produce
slanted text
Character Attributes
Other attributes Character spacing
Spacing between character Alignment
How text is to be displayed with respect to a reference position Align according to the base line Align according to the character center
Text-precision attribute (in some graphics packages) Specify the amount of detail and the particular
processing options that are to be used with the text string
Special character
Antialiasing
Antialiasing
A methods that compensate for the undersampling process (aliasing)
Aliasing Is a distortion of information due to low-frequency
sampling (under-sampling) The sampling process digitizes coordinate points on an
object to discrete integer pixel position Resulting
A jagged, or stair-step appearance
Antialiasing
Antialiasing
One way to increase sampling rate with raster system Simply to display objects at higher resolution Even with highest resolution, the jaggies still apparent
to some extent So it is not a complete solution
Other antialising method may be applied
Antialiasing Methods
Supersampling Straight Line Segments Divide each pixel into a number of subpixels and count the number of
subpixels that overlap the line path. The intensity level is for each pixel is then set to a value that is
proportional to this subpixel count. Involve computing intensities at subpixel grid position, then combine
the results to obtain the pixel intensities Area sampling (prefiltering)
Determine pixel intensity by calculating the areas of overlap of each pixel with the object to be displayed
Pixel phasing Shifting the display location of pixel areas Applied by “micropositioning” the electron beam in relation to object
geometry Filtering techniques (postfiltering)
More accurate method for antialiasing lines Methods similar to applying a weighted pixel mask, but now we
imagine a continuous weighting surface covering the pixel Antialiasing Area Boundary
Applying an antialiasing concepts for boundaries area
