优秀课件笔记之 Attributes of Graphics Primitives

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Chapter 4
Attributes of Graphics Primitives

State System and State Variables

A graphics system that maintain a list for the current values of attributes and parameters is called state system, or state machine.
Attributes of output primitives and some other parameters are called state variables or state parameters.

§4.1 OpenGL State Variables

OpenGL State Variables include:
Color and other primitives attributes
The current matrix mode
The elements of the model-view matrix
The current position for the frame buffer
The parameters for the lighting effects
Etc.

§4.2 Color and Grayscale

RGB color components
R, G, B components are mixed to display a particular color
Described with float numbers, the components ranged from 0.0 to 1.0
Described with integers, the components ranged from 0 to 255
Grayscale: only lighteness, without color
Described with float numbers, the lighteness ranged from 0.0 to 1.0
Described with integers, the lighteness ranged from 0 to 255

§4.3 OpenGL Color Functions

Set the color display mode:

glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
glutInitDisplayMode (GLUT_SINGLE | GLUT_RGBA);

Select the current color:
glColor3f (0.0, 1.0, 1.0);
glColor3i (255, 255, 0);
glColor3fv (colorArray);

RGBA Color Model
In RGBA color model, A is alpha coefficient.
The alpha coefficient controls color blending(颜色调和) for overlapping primitives.
Color blending is used in the simulation of transparency(透明) .

OpenGL Color Blending
To enable and disable color blending:
glEnable (GL_BLEND);
glDisable (GL_BLEND);

§4.4 Point Attributes in OpenGL

In OpenGL, a point has two attributes:
Size and color
To set the size of a point in OpenGL:
glPointSize(size);
To set the color of a point in OpenGL:
glColor3f (1.0, 0.0, 0.0);
glBegin (GL_POINTS);
glVertex2i (50,100);
glPointSize (2.0);
glEnd ( );

§4.5 Line Attributes in OpenGL

In OpenGL, a line has three attributes in general:
Width, style and color

To set the width of a line in OpenGL:

glLineWidth(width);
To set the color of a line in OpenGL, like to set the color of other primitives.

glColor3f (1.0, 0.0, 0.0);
glBegin (GL_LINES);
glVertex2iv (p1);
……
glEnd ( );

To Set Line-Style in OpenGL

To enable and disable line-style function:

glEnable (GL_LINE_STIPPLE);
glDisable (GL_LINE_STIPPLE);
To set the line-style:
glLineStipple (repeatFactor, pattern);
pattern is a 16-bit integer to describe the line-style:
§4.6 Curve Attributes

Parameters of curve attributes are the same as those for straight-line segments.

§4.9 Fill-Area Attributes

Scan-line approach:
Usually used in simple shapes
General graphics packages use this method
Use a starting interior point:
Usually used in complex shapes or interactive systems
Fill Styles

A basic fill-area attribute provided by a general graphics library is the display style of the interior.
We can display a region with
a hollow style
a single color
a specified fill pattern
§4.14 OpenGL Fill-Area Functions

We generate displays of filled convex polygons(凸多边形) in 4 steps:
Define a fill pattern;

Invoke the polygon-fill routine;

Activate the polygon-fill feature of OpenGL;

Describe the polygons to be filled.

GLubyte fillPattern []= {0xff, 0x00, ……};

glPolygonStripple (fillPattern);

glEnable(GL_POLYGON_STIPPLE);

OpenGL Interpolation(插值) Patterns

glShapeModel(GL_SMOOTH);
// GL_SMOOTH means smooth transition(过渡) effect
glBegin(GL_TRANGLES)
glColor(1.0, 0.0, 0.0);
glVertex2i(50,50);
glColor(0.0, 0.0, 1.0);
glVertex2i(0, 0);
glVertex2i(75, 0);
glEnd();
OpenGL Wire-Frame Methods

When the edges and interior area of a same polygon have different colors, we can:
glPolygonModel(GL_FRONT, GL_FILL);
glColor3f(1.0, 0.0, 0.0);
// Invoke polygon-generating routines

glPolygonModel(GL_FRONT, GL_LINE);
glColor3f(0.0, 1.0, 0.0);
// Invoke polygon-generating routines
§4.17 Anti-aliasing (反走样)

Graphics primitives have a jagged(锯齿状) appearance because of the low-frequent sampling(低频采样).
This is called aliasing (走样).
Methods that can eliminate aliasing is called anti-aliasing (反走样).
One is super-sample(过采样).
§4.18 OpenGL Anti-aliasing (反走样) Functions

glEnable (primitiveType);
// primitiveType can be GL_POINT_SMOOTH,
// GL_LINE_SMOOTH, GL_POLYGON_SMOOTH

glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
// Color blending
§4.19 OpenGL Query Functions

Using OpenGL Query Functions, we can get current values for any state parameters.
glGet functions:
Each function needs two parameters:
glGetBooleanv(...);
glGetFloatv(...);
glGetIntegerv(...);
glGetFloatv(GL_CURRENT_COLOR, colorValues);
// The first parameter means what to be queried;
// The second parameter store the queried result.