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Raytracing algorithm

Raytracing is, in many ways, a standardized technique. Almost all raytracing programs (known as "raytracers") use the same basic algorithm in one way or the other. This algorithm is presented below.

The Pre-Rendering Section

This section is executed before the actual rendering of the scene takes place. It is the most implementation and programmer dependent of all the sections of the algorithm. Its structure and content relies entirely on how the programmer has chosen to describe and store his scene and what variables need to be initialized for the program to run properly. Some parts that are usually present in some form or the other include generation of efficiency structures, initialization of the output device, setting up various global variables etc. No standard algorithm exists for this section.

The Main Rendering Loop

This section actually produces the picture of the 3-D scene. The algorithm for this section is given below. Note that the procedure RayTrace is recursive, that is, it calls itself.

Procedure RenderPicture()
  For each pixel on the screen,
    Generate a ray R from the viewing position through the point on the view
      plane corresponding to this pixel.
    Call the procedure RayTrace() with the arguments R and 0
    Plot the pixel in the colour value returned by RayTrace()
  Next pixel
End Procedure
Procedure RayTrace(ray R, integer Depth) returns colour
  Set the numerical variable Dis to a maximum value
  Set the object pointer Obj to null
  For each object in the scene
    Calculate the distance (from the starting point of R) of the nearest
      intersection of R with the object in the forward direction
    If this distance is less than Dis
      Update Dis to this distance
      Set Obj to point to this object
    End if
  Next object
  If Obj is not null
    Set the position variable Pt to the nearest intersection point of R and Obj
    Set the total colour C to black
    For each light source in the scene
      For each object in the scene
        If this object blocks the light coming from the light source to Pt
          Attenuate the intensity of the received light by the transmittivity
            of the object
        End if
      Next object
      Calculate the perceived colour of Obj at Pt due to this light source
        using the value of the attenuated light intensity
      Add this colour value to C
    Next light source
    If Depth is less than a maximum value
      Generate two rays Refl and Refr in the reflected and refracted directions,
        starting from Pt
      Call RayTrace with arguments Refl and Depth + 1
      Add (the return value * reflectivity of Obj) to C
      Call RayTrace with arguments Refr and Depth + 1
      Add (the return value * transmittivity of Obj) to C
    End if
  Else
    Set the total colour C to the background colour
  End if
  Return C
End Procedure

The Post-Rendering Section

This section is also more or less programmer dependent, but the functions performed usually fall into one or more of the following groups

Siddhartha Chaudhuri, 2002