13. Object selection

Transcript

1. 13. Object selection
   Tatsuya Yatagawa
   

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2. Object selection in OpenGL
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   Using selection buffer
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   This is available in legacy OpenGL (without shader), but it is
   rather complicated and unintuitive.
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   Write labels to off-screen buffer
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   If shader program is available, it is more intuitive.
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   Draw a label (i.e., an index to identify the object) on screen.
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   It’s easy! You can do it just by not calling “glfwSwapBuffers.”
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   If target buffer is off-screen buffer, it does not affect visible
   objects!
   Computer Graphics Course @Waseda University 2
   

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3. Workflow to object selection
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   Workflow
   1. Detect mouse click (by mouse event callback).
   2. Write object index as pixel color (to off-screen buffer).
   3. Pick up pixel values using glReadPixels.
   4. Check pixel value and identify selected object.
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4. Write selection label as pixel color
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   Pass selection label as uniform variable
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   Set “selectID” as positive index if in selection mode. Otherwise set -1.
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5. Process in shader programs
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   Vertex shader
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   Nothing is changed. Perform coordinate transformation.
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   Fragment shader
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   Transform “selectID” to floating point number in
   [0, 1] when ID is positive.
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   Note: float outputs are automatically converted to UINT8 by multiplying 255.
   

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6. Pick up pixel values
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   glReadPixels
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   Parameters
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   1st: X coordinate of upper left pixel of buffer area to be read.
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   2nd: Y coordinate of upper left pixel of buffer area to be read.
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   3rd: Width of buffer area to be read.
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   4th: Height of buffer area to be read.
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   5th: Color format of buffer data (typically GL_RGBA).
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   6th: Scalar type of buffer data (typically GL_UNSIGNED_BYTE).
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   7th: Pointer to data array in which the read data is stored.
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7. Pick up pixel value(s)
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   Straightforward approach
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   Read entire buffer, and pick up its value at clicked pixel.
   Problem
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   Data transfer between RAM and GPU memory is time consuming.
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   Reading entire buffer is not efficient.
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   Be careful that Y coordinate is inversed!
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8. Pick up pixel value(s)
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   More efficient approach
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   Only read value at clicked pixel!
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9. Whole code for object selection
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10. Result
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    Object index is printed when each cube is clicked
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11. Exercise 13-1
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    Select cube faces rather than whole cube. Then,
    highlight only the selected face. When non-cube region
    is clicked, highlight entire cube.
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    You should draw only a single cube (do not have to draw
    multiple cubes as in the example).
    Computer Graphics Course @Waseda University 11
    

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