Cast Vector3[] to float3[]

Question

Hey there,

I'm developing a critical part of an application that must iterate through an array of 1.000.000+ Vector3 objects. Through some testing I discovered that the speed of the existing algorithm improves 30% just by iterating through a float3 array instead.

After manipulation, the array must feed a mesh vertices array, so must be converted to Vector3[].

As the distribution in memory of a Vector3 and a float3 is the same, I know there must be a way to quickly cast from one to the other (in C it would be through pointers, but doesn't seem as easy in C+).

I also tried to copy from Vector3[] to float3[], but obviously this method makes me have a duplication in memory. The way I tried to do this is with the following:

     [DllImport("kernel32.dll", EntryPoint = "CopyMemory", SetLastError = false)]
     private static unsafe extern void CopyMemory(void* dest, void* src, int count);

     private static unsafe void Serialize(Vector3[] src, float3[] dst)
     {
         fixed (void* d = &dst[0])
         {
             fixed (void* s = &src[0])
             {
                 CopyMemory(d, s, src.Length * sizeof(float) * 3);
             }
         }
     }

Anybody has an idea on how to achieve this?

Answer 1

Answer by andrew-lukasik · Mar 03, 2021 at 09:19 AM

 using Unity.Collections;
 using Unity.Collections.LowLevel.Unsafe;
 using Assert = UnityEngine.Assertions.Assert;
 
 public unsafe static void MemCpy <SRC,DST> ( SRC[] src , DST[] dst )
     where SRC : struct
     where DST : struct
 {
     int srcSize = src.Length * UnsafeUtility.SizeOf<SRC>();
     int dstSize = dst.Length * UnsafeUtility.SizeOf<DST>();
     Assert.AreEqual( srcSize , dstSize , $"{nameof(srcSize)}:{srcSize} and {nameof(dstSize)}:{dstSize} must be equal." );
     void* srcPtr = UnsafeUtility.PinGCArrayAndGetDataAddress( src , out ulong srcHandle );
     void* dstPtr = UnsafeUtility.PinGCArrayAndGetDataAddress( dst , out ulong dstHandle );
     UnsafeUtility.MemCpy( destination:dstPtr , source:srcPtr , size:srcSize );
     UnsafeUtility.ReleaseGCObject( srcHandle );
     UnsafeUtility.ReleaseGCObject( dstHandle );
 }
 
 public unsafe static void MemCpy <SRC,DST> ( NativeArray<SRC> src , DST[] dst )
     where SRC : struct
     where DST : struct
 {
     int srcSize = src.Length * UnsafeUtility.SizeOf<SRC>();
     int dstSize = dst.Length * UnsafeUtility.SizeOf<DST>();
     Assert.AreEqual( srcSize , dstSize , $"{nameof(srcSize)}:{srcSize} and {nameof(dstSize)}:{dstSize} must be equal." );
     void* srcPtr = NativeArrayUnsafeUtility.GetUnsafeReadOnlyPtr( src );
     void* dstPtr = UnsafeUtility.PinGCArrayAndGetDataAddress( dst , out ulong handle );
     UnsafeUtility.MemCpy( destination:dstPtr , source:srcPtr , size:srcSize );
     UnsafeUtility.ReleaseGCObject( handle );
 }

< credits roll >

unity3d player settings allow unsafe code tutorial

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pelaezjorge · Mar 03, 2021 at 10:33 PM 0

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Thank you for your answer! I'm sure that it can be helpful in some situations, but it copies memory, so we still have a duplication, right? The good point of this over the method I use, is that this one is not Windows-dependent.

andrew-lukasik pelaezjorge · Mar 04, 2021 at 12:28 AM 0

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Yes, you need two allocations of matching lengths but this copy operation is one of the fastest possible. Definitely faster that what Linq's Cast() bs can do.

You can cast pointers tho

 float3 f3 = new float3{};
 float3* f3Ptr = &f3;
 Vector3* v3ptr = (Vector3*) f3Ptr;

I don't think you can cast struct array references in c# right now.

andrew-lukasik pelaezjorge · Mar 04, 2021 at 12:56 AM 0

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Btw. try this instead: Cast every value before/after processing and ignore all that array type casting drama:

 Vector3[] vectors = ...;
 for( int i=0 ; i<vectors.Length ; i++ )
 {
     float3 f3 = (float3) vectors[i];// cast #1
     {
         // all relevent data transformations
         f3 = math.pow( f3 , 10 );
     }
     vectors[i] = (Vector3) f3;// cast #2
 }

Cast Vector3[] to float3[]

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