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How to …

How to …. Establish your position in the scene Position objects within the scene Scale objects Establish a perspective transformation Moving around in OpenGL using a camera. Terminology. The Pipeline. Eye Coordinates. Viewpoint of the observer They are a virtual fixed coordinate system

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How to …

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  1. How to … • Establish your position in the scene • Position objects within the scene • Scale objects • Establish a perspective transformation • Moving around in OpenGL using a camera

  2. Terminology

  3. The Pipeline

  4. Eye Coordinates • Viewpoint of the observer • They are a virtual fixed coordinate system • Used as a common frame of reference

  5. Viewing Transformations • Like placing and pointing the camera at the scene • To be specified before any other transformation • By default, the point of observation is at the origin, looking down the negative Z-axis. What about objects drawn with positive Z values?

  6. Modeling Transformation • Manipulate the model and the particular objects within it • Move objects into place, rotate them, and scale them • These are the most common modeling transformations

  7. Modeling Transformation - the ORDER matters • Final appearance of an object depends greatly on the order of transformations

  8. ModelView Duality • In fact, viewing and modeling transformations are the same • Induce the same effects on the final appearance of the scene • The distinctions is made as a convenience for the programmer

  9. Projection Transformations • Applied after the modelview transformation • Defines the viewing volume and the clipping planes • Specifies how a finished scene is projected to the screen

  10. Orthographic Vs. Perspective • In an Orthographic projection: • objects are mapped directly on the 2D screen using parallel lines • No matter how far away something is • In Perspective projection: • Scenes are more realistic • Distant objects appear smaller than nearby objects of the same size • Parallel lines in 3D space do not always appear parallel See Code Example 3

  11. The Pipeline - Recall

  12. Slow but steady .. • Current Matrix is part of the OpenGL state • Options are: • GL_MODELVIEW – changes, rotate, translate the model • GL_PROJECTION – changes, translate, rotate the camera • To specify which one is the “Current Matrix” :glMatrixMode(…) • Subsequent matrix operations affect the specified matrix • One of this matrices could be change in a given time

  13. Translation void glTranslatef(GLfloat x, GLfloat y, GLfloat z ) This function takes as parameters the amount to translate along the x, y, and z directions

  14. Translation – (cont’) glTranslate(0.0f, 10.0f, 0.0f) glutWireCube(10.0f) • Construct a translation matrix for positive 10 Y • Multiply it by themodelview matrix

  15. Rotation void glRotatef(Glfloatangle,GLfloat x, GLfloat y, GLfloat z ) This function performs a rotation around the vector specified by the x, y, and z arguments. The angle of rotation is in the counterclockwise direction. A rotation around an arbitrary axis could be done.

  16. Scaling void glScalef(GLfloat x, GLfloat y, GLfloat z ) Multiplies the x, y and z values by the scaling factors specified. Scaling does not have to be uniform.

  17. The Identity Matrix • Matrix operations are cumulative • Erroneous artifacts may occur to the scene • You reset the origin by loading the modelview matrix with the identity matrix glTranslate(0.0f, 10.0f, 0.0f); glutSolidSphere(1.0f); glTranslatef(10.0f, 0.0f, 0.0f); glutSolidSphere(1.0f)

  18. The Identity Matrix – (cont’) glMatrixMode(GL_MODELVIEW) glLoadIdentity(); glTranslate(0.0f, 10.0f, 0.0f); glutSolidSphere(1.0f); glLoadIdentity(); glTranslatef(10.0f, 0.0f, 0.0f); glutSolidSphere(1.0f)

  19. Matrix stack • Resetting the modelview matrix to identity before placing the every object is not always desirable • To save the current transformation state • OpenGL maintains a matrix stack for both the modelview and projection matrices • glPushMatrix() – push the current state matrix to the stack • glPopMatrix() – pop the top matrix out of the stack, andreplace the current state matrix with it

  20. Matrix stack – (cont’) See Code Example 4

  21. Viewing Volume glOrtho(left, right, bottom, top, near, far)

  22. Viewing Volume – (cont’) glFrustum(left, right, bottom, top, near, far) gluPerspective(GLdoublefov ,GLdoubleaspectRatio,GLdoublezNear,GLdoublezFar)

  23. Camera Management gluLookAt(GLdoubleeyex, GLdoubleeyey, Gldoubleeyez, GLdoublecenterx, GLdoublecentery, GLdoublecenterz, GLdoubleupx, GLdoubleupy, GLdoubleupz)

  24. Buffers • Important aspect of the current state of OpenGL • Buffers dimensions are the same as the window’s dimension • ColorBuffer – stores the current color for each pixel • Depth Buffer – stores the depth of each pixel of the scene (Z-Buffer) • Accumulation buffer, stencil buffer, etc’ .. • glEnable(GL_DEPTH_TEST) – enables the Z-Buffer in OpenGL. ….. Otherwise?

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