CS 551 / 645: Introductory Computer Graphics

1. CS 551 / 645: Introductory Computer Graphics David Luebke cs551@cs.virginia.edu http://www.cs.virginia.edu/~cs551 David Luebke 8/28/2014

2. Recap: OpenGL • OpenGL provides an interface and implementation for interactive rendering. • It has become a standard because: • A standard was badly needed • OpenGL is pretty good • SGI promoted it and Microsoft (sorta) bought in • OpenGl is particularly tuned to hardware-accelerated transformation, lighting, texturing, and Z-buffering David Luebke 8/28/2014

3. Recap: OpenGL Conventions • Functions in OpenGL start with gl (or glu) • Function names indicate argument type/# • E.g., glColor3f() vs glColor3fv() vs glColor4ub() • Geometry is specified as a list of vertices between glBegin() and glEnd() calls: glBegin(GL_POLYGON); glVertex3f(x1, y1, z1); glVertex3f(x2, y2, z2); glVertex3f(x3, y3, z3); glEnd(); David Luebke 8/28/2014

4. v2 v4 v0 v5 v1 v3 Recap: Miscellaneous OpenGL • The vertices of the front side of a polygon are ordered counterclockwise • You can draw multiple triangles between glBegin(GL_TRIANGLES) and glEnd() • The GL_TRIANGLE_STRIPprimitive reduces redundancy by sharing vertices: David Luebke 8/28/2014

5. Miscellaneous OpenGL • The GL_TRIANGLE_FAN primitive is another way to reduce vertex redundancy: v4 v3 v5 v0 v2 v6 v1 David Luebke 8/28/2014

6. Recap: OpenGL Lighting • OpenGL binds our Phong lighting coefficients (ka, kd, ks, nshiny) into materials • Calling glMaterialfv() sets a single attribute of the current material • Example: float green[] = {0, 1, 0, 1}; float white[] = {1, 1, 1, 1}; glMaterialfv(GL_FRONT, GL_DIFFUSE, green); glMaterialfv(GL_FRONT, GL_SPECULAR, white); David Luebke 8/28/2014

7. Recap: OpenGL Lighting • OpenGL supports at least 8 lights, with parameters set by the glLight() call: float l_amb [] = {.1, .1, .1, 1.0}; float l_diff[] = {1, 0, 0, 1}; float l_spec[] = {1, 1, 1, 1}; float l_pos[] = {10, 100, 30, 0}; glLightfv(GL_LIGHT0, GL_AMBIENT, l_amb); glLightfv(GL_LIGHT0, GL_DIFFUSE, l_diff); glLightfv(GL_LIGHT0, GL_SPECULAR, l_spec); glLightfv(GL_LIGHT0, GL_POSITION, l_pos); (the 4th coordinate in l_pos is 0.0 for a directional light, 1.0 for a point light) David Luebke 8/28/2014

8. Errata: OpenGL Lighting • Recall the Phong lighting model: • OpenGL modifies this slightly: • Each light contributes separately to ambient term • Lights have a separate intensity for specular reflection (Why might this be useful?)

9. OpenGL: Lighting • Don’t forget to enable lighting and each light: glEnable(GL_LIGHTING); glEnable(GL_LIGHT0); • Can set the lighting model too: • Intensity of the ambient light source • Whether to treat eye point as infinitely far away • Whether to perform lighting calculations for both sides of polygons • All these things have reasonable default values • man glLightModel for details David Luebke 8/28/2014

10. OpenGL: Display Lists • OpenGL can “compile” a series of rendering commands into a display list... glNewList(1, GL_COMPILE); glBegin(GL_TRIANGLES); glVertex3fv(v0); /* draw lots of triangles… */ glVertex3fv(v2); glEnd(); glEndList(); • …which can be rendered with a single call: glCallList(1); David Luebke 8/28/2014

11. OpenGL: Display Lists • Display lists can contain: • Geometry • Color, material, and texture specifications • Matrix transforms (up shortly) • Other display lists! • Display lists are not only handy, they usually increase performance • Why might OpenGL be able to render a series of commands faster if they have been compiled into a display list? David Luebke 8/28/2014

12. OpenGL: Matrix Manipulation • OpenGL keeps two matrices that vertices are multiplied by upon calling glVertex() • The modelview matrix combines all modeling transforms and the viewing transform • The projection matrix performs the projection (usually a perspective projection) • Various commands affect the current matrix • You need to specify which matrix is current: • E.g., glMatrixMode(GL_MODELVIEW) or glMatrixMode(GL_PROJECTION) • glLoadIdentity() replaces the contents of the current matrix with the identity matrix David Luebke 8/28/2014

13. OpenGL: Modeling Transforms • Some OpenGL commands generate transformation matrices: glTranslatef(Tx, Ty, Tz); glRotatef(angleDegrees, Ax, Ay, Az); glScalef(Sx, Sy, Sz); • The resulting matrix concatenates to the right of the current matrix David Luebke 8/28/2014

14. OpenGL: Modeling Transforms • Example: glMatrixMode(GL_MODELVIEW); glLoadIdentity(); glTranslatef(…); glRotatef(…); • Result: the modelview matrix is set to: I • T • R == T • R which then multiplies all following vertices • Thus transformations appearing last in the program have the firsteffect on the geometry David Luebke 8/28/2014

15. OpenGL: Viewing Transforms • Viewing transforms are treated the same way • Ex: gluLookAt() computes a lookat matrix and concatenates it with the current matrix: gluLookAt(eyeX, eyeY, eyeZ, centerX, centerY, centerZ, upX, upY, upZ); • Again, this matrix postmultiplies the current matrix • ShouldgluLookAt()be called first or last? David Luebke 8/28/2014

16. OpenGL: Projection Transforms • The projection matrix is generally used for the perspective projection matrix • Why do you suppose OpenGL separates the modelview and projection matrices? • gluPerspective() creates a projection matrix similarly to the call in assignment 6 gluPerspective(double FOVy, double aspect, double near, double far); • FOVy: field of view (°) in the y vertical (y) direction aspect: viewport width (y) divided by height (x) David Luebke 8/28/2014

17. The Scene Graph • Recall the concept of instancing, or using the same geometry for multiple objects • Example: 4 wheels on car • How might we use display lists for instancing? • Compile geometry (say a car tire, centered about the origin) into a display list • Set up matrices: viewing transform + modeling transform(s) to put origin at front left corner of car • Call display list for tire • Set up matrices, this time putting origin at front right of car • Call display list for tire [Etc…] • Why is this inefficient? David Luebke 8/28/2014

18. The Scene Graph • Answer: because many objects in a scene typically share multiple transformations • The scene graph captures transformations and object-object relationships in a DAG: World Objects Robot Instancing(i.e, a matrix) Head Body Legend Mouth Eye Leg Trunk Arm David Luebke 8/28/2014

19. The Scene Graph • Traverse the scene graph in depth-first order, concatenating and undoing transforms: • For example, to render the robot: • Apply robot head matrix • Apply head mouth matrix • Render mouth • Un-apply head mouth matrix • Apply head left eye matrix • Render eye • Un-apply head left eye matrix • Apply head right eye matrix • Render eye • Un-apply head right eye matrix • Un-apply robot head matrix • Apply robot body matrix How should we implement this“un-apply” business? David Luebke 8/28/2014

20. The Scene Graph in OpenGL • OpenGL maintains a matrix stack of modeling and viewing transformations: Robot Visited Head Body Unvisited Leg Mouth Eye Trunk Arm MatrixStack Active Foot David Luebke 8/28/2014

21. OpenGL: The Matrix Stack • The user can save the current transformation matrix by pushing it onto the stack with glPushMatrix() • The user can later restore the most recently pushed matrix with glPopMatrix() • These commands really only make sense when in GL_MODELVIEW matrix mode David Luebke 8/28/2014

22. OpenGL: Matrix Stack Example glMatrixMode(GL_MODELVIEW); glLoadIdentity(); glTranslatef(…); // save translation matrix: glPushMatrix(); glRotatef(…); // render something translated & rotated: glCallList(foo); // restore pushed matrix, undoing rotation: glPopMatrix(); // render something else, no rotation: glCallList(bar); David Luebke 8/28/2014

23. Coming Up: • Animation: smooth (flicker-free) motion using double buffering • More OpenGL tricks • Backface culling • Gouraud shading • Computing vertex normals David Luebke 8/28/2014