The GetLandscapeBelowVertex subroutine - Lander on the Acorn Archimedes

       Name: GetLandscapeBelowVertex                                 [Show more]
       Type: Subroutine
   Category: Landscape
    Summary: Calculate the landscape altitude directly below an object's vertex
  Deep dive: Generating the landscape
             Drawing 3D objects
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DrawObject (Part 2 of 5) calls GetLandscapeBelowVertex


 Arguments:

   R0                               The address containing the object's vertex
                                    (x, y, z), relative to the camera position


 Returns:

   R0                               The altitude (y-coordinate) of the landscape
                                    directly below the coordinate


.GetLandscapeBelowVertex

 STMFD   R13!, {R8-R9, R14}         \ Store the registers that we want to use on
                                    \ the stack so they can be preserved

 LDR     R8, [R0]                   \ Add xCamera to the x-coordinate in R8 to
 LDR     R1, [R11, #xCamera]        \ get the vertex position in the game's
 ADD     R8, R8, R1                 \ world coordinate system

 LDR     R9, [R0, #8]               \ Add zCamera to the z-coordinate in R8 to
 LDR     R1, [R11, #zCamera]        \ get the vertex position in the game's
 ADD     R9, R9, R1                 \ world coordinate system

 SUB     R9, R9, #LANDSCAPE_Z       \ Move the z-coordinate forward by the
                                    \ landscape offset, as the altitude
                                    \ calculation needs the coordinate to be
                                    \ relative to the front-centre point of the
                                    \ landscape

                                    \ The (x, z) coordinate in (R8, R9) is now
                                    \ relative to the game's coordinate system,
                                    \ rather than the camera or the landscape
                                    \ offset, which is what we need in order
                                    \ to calculate the altitude of the landscape
                                    \ at this point

 BL      GetLandscapeAltitude       \ Set R0 to the altitude of the landscape at
                                    \ coordinates (x, z) = (R8, R9), which is
                                    \ the point directly below the vertex

 LDR     R14, [R11, #yCamera]       \ Subtract yCamera from the altitude so the
 SUB     R0, R0, R14                \ result is relative to the camera position

 LDMFD   R13!, {R8-R9, PC}          \ Retrieve the registers that we stored on
                                    \ the stack and return from the subroutine

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Subroutine GetLandscapeAltitude (category: Landscape)

Calculate the altitude of the landscape for a given coordinate

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Configuration variable LANDSCAPE_Z = LANDSCAPE_Z_DEPTH + (10 * TILE_SIZE)

The z-coordinate of the landscape offset, which is set to push the landscape away from the viewer by ten tile sizes

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Configuration variable xCamera = &13C

The 3D x-coordinate of the camera position (though note that the camera position is actually at the back of the on-screen landscape, not the front)

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Configuration variable yCamera = &140

The 3D y-coordinate of the camera position (though note that the camera position is actually at the back of the on-screen landscape, not the front)

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Configuration variable zCamera = &144

The 3D z-coordinate of the camera position (though note that the camera position is actually at the back of the on-screen landscape, not the front, so the camera's z-coordinate is larger than it would be for a more traditional camera position; it is more like the camera's focal point than position, in a sense)