Mumbai university > Comp > SEM 4 > Computer Graphics

Marks: 10M

Year: Dec 2014

1. OpenGL is the software interface to the graphic hardware.

2. OpenGL's main purpose is to render two- and three-dimensional objects into a frame buffer.

3. These objects are described as sequences of vertices (which define geometric objects) or pixels (which define images).

4. OpenGL performs several processing steps on this data to convert it to pixels to form the final desired image in the frame buffer.

5. OpenGL can render:

   • Geometric primitives such as Lines, points, polygons, etc.
   • Bitmaps and Images

OpenGL Primitives:

Point:

GL_POINTS:

• Treats each vertex as a single point.

• Vertex n defines a point n.

• N points are drawn.

• Sample:

$$glBegin(GL_POINTS);$$

$$glVertex2f(x1, y1);$$

$$glEnd();$$

Line:

GL_LINES:

• Treats each pair of vertices as an independent line segment.

• Vertices 2n-1 and 2n define a line n.

• N/2 lines are drawn.

• Sample:

$$glBegin(GL_LINES);$$

$$glVertex2f(x1, y1);$$

$$glVertex2f(x2, y2);$$

$$glEnd();$$

GL_LINE_STRIP:

• Draws a connected group of line segments from the first vertex to the last.

• Vertices n and n+1 define line n.

• N-1 lines are drawn.

• Sample:

$$glBegin(GL_LINE_STRIP);$$

$$glVertex2f(x1, y1);$$

$$glVertex2f(x2, y2);$$

$$glVertex2f(x3, y3);$$

$$glEnd();$$

GL_LINE_LOOP:

• Draws a connected group of line segments from the first vertex to the last, then back to the first.

• Vertices n and n+1 define line n.

• N lines are drawn.

• Sample:

$$glBegin(GL_LINE_LOOP);$$

$$glVertex2f(x1, y1);$$

$$glVertex2f(x2, y2);$$

$$glVertex2f(x3, y3);$$

$$glEnd();$$

Polygon:

GL_TRIANGLES:

• Treats each triplet of vertices as an independent triangle.

• Vertices 3n-2, 3n-1, and 3n define triangle n.

• N/3 triangles are drawn.

• Sample:

$$glBegin(GL_TRIANGLES);$$

$$glVertex2f(x1, y1);$$

$$glVertex2f(x2, y2);$$

$$glVertex2f(x3, y3);$$

$$glEnd();$$

GL_TRIANGLE_STRIP:

• Draws a connected group of triangles.

• One triangle is defined for each vertex presented after the first two vertices.

• For odd n, vertices n, n+1, and n+2 define triangle n.

• For even n, vertices n+1, n, and n+2 define triangle n.

• N-2 triangles are drawn.

• Sample:

$$glBegin(GL_TRIANGLE_STRIP);$$

$$glVertex2f(x1, y1);$$

$$glVertex2f(x2, y2);$$

$$glVertex2f(x3, y3);$$

$$glVertex2f(x4, y4);$$

$$glEnd();$$

GL_TRIANGLE_FAN:

• Draws a connected group of triangles that fan around a central point.

• One triangle is defined for each vertex presented after the first two vertices.

• Vertices 1, n+1, and n+2 define triangle n.

• N-2 triangles are drawn.

• Sample:

$$glBegin(GL_TRIANGLE_FAN);$$

$$glVertex2f(x1, y1);$$

$$glVertex2f(x2, y2);$$

$$glVertex2f(x3, y3);$$

$$glVertex2f(x4, y4);$$

$$glEnd();$$

GL_QUADS:

• Treats each group of four vertices as an independent quadrilateral.

• Vertices 4n-3, 4n-2, 4n-1, and 4n define quadrilateral n.

• N/4 quadrilaterals are drawn.

• Symbol:

$$glBegin(GL_QUADS);$$

$$glVertex2f(x1, y1);$$

$$glVertex2f(x2, y2);$$

$$glVertex2f(x3, y3);$$

$$glVertex2f(x4, y4);$$

$$glEnd();$$

GL_QUAD_STRIP:

• Draws a connected group of quadrilaterals.

• One quadrilateral is defined for each pair of vertices presented after the first pair.

• Vertices 2n-1, 2n, 2n+2, and 2n+1 define quadrilateral n.

• N/2-1 quadrilaterals are drawn.

• Symbol:

$$glBegin(GL_QUAD_STRIP);$$

$$glVertex2f(x1, y1);$$

$$glVertex2f(x2, y2);$$

$$glVertex2f(x3, y3);$$

$$glVertex2f(x4, y4);$$

$$glVertex2f(x5, y5);$$

$$glVertex2f(x6, y6);$$

$$glEnd();$$

GL_POLYGON:

• Draws a single, convex polygon.

• Vertices 1 through N define this polygon.

• A polygon is convex if all points on the line segment between any two points in the polygon or at the boundary of the polygon lie inside the polygon.

• Symbol:

$$glBegin(GL_POLYGON);$$

$$glVertex2f(x1, y1);$$

$$glVertex2f(x2, y2);$$

$$glVertex2f(x3, y3);$$

$$glVertex2f(x4, y4);$$

$$glVertex2f(x5, y5);$$

$$glEnd();$$