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72 lines
2.0 KiB
Common Lisp
72 lines
2.0 KiB
Common Lisp
// Calculate the first derivative from a 3x3 cell matrix
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float calcFirstDer( float x11, float x21, float x31, float x12, float x22, float x32, float x13, float x23, float x33,
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float inputNodataValue, float outputNodataValue, float zFactor, float mCellSize )
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{
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//the basic formula would be simple, but we need to test for nodata values...
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//X: return (( (x31 - x11) + 2 * (x32 - x12) + (x33 - x13) ) / (8 * cellSizeX));
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//Y: return (((x11 - x13) + 2 * (x21 - x23) + (x31 - x33)) / ( 8 * cellSizeY));
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int weight = 0;
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float sum = 0;
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//first row
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if ( x31 != inputNodataValue && x11 != inputNodataValue ) //the normal case
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{
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sum += ( x31 - x11 );
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weight += 2;
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}
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else if ( x31 == inputNodataValue && x11 != inputNodataValue && x21 != inputNodataValue ) //probably 3x3 window is at the border
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{
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sum += ( x21 - x11 );
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weight += 1;
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}
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else if ( x11 == inputNodataValue && x31 != inputNodataValue && x21 != inputNodataValue ) //probably 3x3 window is at the border
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{
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sum += ( x31 - x21 );
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weight += 1;
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}
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//second row
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if ( x32 != inputNodataValue && x12 != inputNodataValue ) //the normal case
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{
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sum += 2.0f * ( x32 - x12 );
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weight += 4;
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}
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else if ( x32 == inputNodataValue && x12 != inputNodataValue && x22 != inputNodataValue )
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{
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sum += 2.0f * ( x22 - x12 );
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weight += 2;
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}
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else if ( x12 == inputNodataValue && x32 != inputNodataValue && x22 != inputNodataValue )
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{
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sum += 2.0f * ( x32 - x22 );
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weight += 2;
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}
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//third row
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if ( x33 != inputNodataValue && x13 != inputNodataValue ) //the normal case
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{
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sum += ( x33 - x13 );
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weight += 2;
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}
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else if ( x33 == inputNodataValue && x13 != inputNodataValue && x23 != inputNodataValue )
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{
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sum += ( x23 - x13 );
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weight += 1;
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}
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else if ( x13 == inputNodataValue && x33 != inputNodataValue && x23 != inputNodataValue )
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{
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sum += ( x33 - x23 );
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weight += 1;
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}
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if ( weight == 0 )
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{
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return outputNodataValue;
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}
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return sum / ( weight * mCellSize ) * zFactor;
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}
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