14. Arcball control

Transcript

1. 14. Arcball control
   Tatsuya Yatagawa
   

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2. What is arcball control?
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   Intuitive control for object transformation
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   Intuitively “rotate” objects by mouse dragging.
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   It is often combined with translation and scaling controls.
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   It is commonly used in CAD and DCC (digital content creation)
   software such as Maya.
   Computer Graphics Course @Waseda University
   Default Rotation Translation Scaling
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3. Arcball control algorithm
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   How can mouse drag be converted to object translation?
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   Mouse drag → motion in “screen space”
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   Object translation → motion in “object space”
   → Inverse translation from screen space to object space
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   Object space
   coordinate
   World space
   coordinate
   Camera space
   coordinate
   Screen space
   coordinate
   Mouse drag
   Object translation
   Model matrix View matrix Projection matrix
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4. Rotation by arcball control
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   Parameters required for rotation
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   Axis of rotation
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   Angle of rotation
   → They can be computed from motion on “arcball sphere”
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   Screen and arcball sphere
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   Required rotation translate red vector to blue vector
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5. Rotation by arcball control
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   Get unit vector to arcball sphere (getVector)
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6. Rotation by arcball control
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   Compute axis and angle in object space
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   * “4.0” is arbitrary scaling (for comfortable control)
   

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7. Remark
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   Is arcball sphere in screen space?
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   Inverse perspective projection distort sphere.
   → For intuitive control, assume arcball is in camera space.
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   引用: コンピュータグラフィックス [改訂新版] (コンピュータグラフィックス編集委員会 著) P. 43
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8. Translation by arcball control
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   Parameters for translation
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   3D vector for object translation in object space.
   → Translate two endpoints of mouse motion to object space.
   Then take their difference.
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   Notice
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   You shouldn’t take difference of screen space positions because
   depth information is lost!
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   Alternatively, you should first transform two screen space
   positions back to world space, then take difference!
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9. Translation by arcball control
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   Get depth value at clicked pixels
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   Depth information can be read with glReadPixels.
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   For simplicity, gravity center of the object is used instead in
   sample program.
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   Code snippet
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10. Translation by arcball control
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    Transformation from screen to object space
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11. Scaling by arcball control
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    Parameters for scaling
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    Ratio of scaling
    → Calculate the ratio from amount of mouse motion.
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    This time, it’ll be calculated by mouse wheel or mouse drag
    during middle button press.
    Computer Graphics Course @Waseda University 11
    

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12. Register mouse wheel event
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    glfwSetScrollCallback
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    Callback for mouse wheel
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    Its parameters represent amount of wheel (not mouse position)
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    Y coordinate corresponds to ordinary mouse wheel.
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    New
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13. Scaling by arcball control
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    Scaling control by dragging with middle button press
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    Calculate the ratio from horizontal mouse move.
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    In sample program, scale up when mouse move to upward.
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    Scaling update
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14. Switch controlled parameters by mouse button
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    Role for each button (changed arbitrarily)
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    Left click → Rotation
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    Right click → Translation
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    Middle button / wheel → Scaling
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    Code snippet
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15. Switch controlled parameters by mouse button
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    Process for mouse click
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    Enable arcball control.
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    Keep click position as a variable.
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16. Switch controlled parameters by mouse button
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    Process for mouse move
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    Update mouse click position.
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    Perform transformation by arcball control.
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17. Switch controlled parameters by mouse button
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    Call corresponding update method for each mode
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18. Switch controlled parameters by mouse button
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    Process for mouse release
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    Initialize mouse click positions to avoid unexpected behavior.
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    Disable arcball control.
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19. Result
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    Arcball control allow us intuitive object transformation!
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    Default Rotation Translation Scaling
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20. Exercise 14-1
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    Implement a program in which only the selected object
    can be controlled using arcball control.
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    Prepare two simple cubes to be controlled.
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    You should prepare an object class which manages translation
    parameters for each object.
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