3D geometry: SetMatrixEntry

       Name: SetMatrixEntry                                          [Show more]
       Type: Subroutine
   Category: 3D geometry
    Summary: Calculate a matrix entry
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * SetMatrices calls SetMatrixEntry
             * SetMatrices calls via SetMatrixEntry2
             * SetMatrices calls via SetMatrixEntry3


 This routine reads one of the projected coordinate values that we calculated
 in ProjectAxisAngle (in mx1 to mz2), performs a multiplication, and writes the
 result to the specified entry in matrix 1.

 If mRead is the matrix entry in X (from mx1 to mz2) and mWrite is the matrix
 entry in Y (from m0 to m8), then it calculates the following:

   mWrite = (J I) * mRead >> 16


 Arguments:

   X                    The matrix entry to read (mRead):

                           * 0 = mx1

                           * 1 = my1

                           * 2 = mz1

                           * 3 = mx2

                           * 4 = my2

                           * 5 = mz2

   Y                    The matrix entry to write (mWrite) in matrix 1:

                           * 0 = m0

                           * 1 = m1

                           * 2 = m2

                             ...

                           * 7 = m7

                           * 8 = m8

   matrixNumber         The routine is only ever called with matrixNumber = 0,
                        so it only ever writes to matrix 1, but calling the
                        routine with a different value would allow us to write
                        to a different matrix

   matrixAxis           The routine is only ever called with matrixAxis = 0,
                        which has no effect on the calculation, but calling the
                        routine with a non-zero value of matrixAxis would allow
                        different mRead values to be used, i.e. matrixAxis+X


 Returns:

   mWrite               Set to (J I) * mRead >> 16

   (S R)                The value of mRead

   (H G)                The value written to mWrite


 Other entry points:

   SetMatrixEntry2      Set mWrite = (S R) * mRead >> 16 and (J I) = mRead

   SetMatrixEntry3      Set mWrite = (J I) * -mRead >> 16 and (S R) = -mRead


.SetMatrixEntry

 TXA                    \ Set X = matrixAxis + X
 CLC                    \
 ADC matrixAxis         \ This has no effect when matrixAxis = 0, which is the
 TAX                    \ only value that this routine is called with

 LDA mx1Lo,X            \ Set R = the low byte from the matrix entry to read
 STA R

.smen1

 LDA mx1Hi,X            \ Set (S R) = the matrix entry to read
 STA S

 JMP smen2              \ Jump down to smen2 to do the calculation

.SetMatrixEntry3

 TXA                    \ Set X = matrixAxis + X
 CLC                    \
 ADC matrixAxis         \ This has no effect when matrixAxis = 0, which is the
 TAX                    \ only value that this routine is called with

 LDA mx1Lo,X            \ Set R = the low byte from the matrix entry to read,
 EOR #1                 \ with the sign in bit 0 flipped
 STA R

 JMP smen1              \ Jump up to smen1 to set the high byte

.SetMatrixEntry2

 TXA                    \ Set X = matrixAxis + X
 CLC                    \
 ADC matrixAxis         \ This has no effect when matrixAxis = 0, which is the
 TAX                    \ only value that this routine is called with

 LDA mx1Lo,X            \ Set (J I) = the matrix entry to read
 STA I
 LDA mx1Hi,X
 STA J

.smen2

 TYA                    \ Set N = Y + matrixNumber
 CLC                    \
 ADC matrixNumber       \ This sets N = Y when matrixNumber = 0, which is the
 STA N                  \ only value that this routine is called with

 JSR Multiply16x16Bit0  \ Set (H G) = (J I) * (S R) >> 16

 LDY N                  \ Fetch the index of the matrix entry to write that we
                        \ stored in N above

 LDA G                  \ Store (H G) in the matrix entry to write
 STA matrix1Lo,Y
 LDA H
 STA matrix1Hi,Y

 RTS                    \ Return from the subroutine