CreateVertexDeclaration error in Delphi

Question

I have simple declarations:

uses ...
  Winapi.Direct3D9, Winapi.DXTypes, Winapi.D3DX9,

var
  g_pVertexDeclaration: IDirect3DVertexDeclaration9 = nil;
const
  decl: array [0 .. 4] of TD3DVertexElement9 = (
    (Stream: 0; Offset: 0; _Type: D3DDECLTYPE_FLOAT3; Method: D3DDECLMETHOD_DEFAULT;
    Usage: D3DDECLUSAGE_POSITION; UsageIndex: 0),
    (Stream: 0; Offset: 3 * sizeof(single); _Type: D3DDECLTYPE_FLOAT3; Method: D3DDECLMETHOD_DEFAULT;
    Usage: D3DDECLUSAGE_NORMAL; UsageIndex: 0),
    (Stream: 0; Offset: 6 * sizeof(single); _Type: D3DDECLTYPE_FLOAT2; Method: D3DDECLMETHOD_DEFAULT;
    Usage: D3DDECLUSAGE_TEXCOORD; UsageIndex: 0),
    (Stream: 0; Offset: 8 * sizeof(single); _Type: D3DDECLTYPE_FLOAT2; Method: D3DDECLMETHOD_DEFAULT;
    Usage: D3DDECLUSAGE_TEXCOORD; UsageIndex: 0),
    (Stream: $FF; Offset: 0; _Type: D3DDECLTYPE_UNUSED; Method: TD3DDeclMethod(0);
    Usage: TD3DDeclUsage(0); UsageIndex: 0) //({D3DDECL_END})
    );

function to log errors

function CheckDxError(e: dword): boolean;
begin
  if Failed(e) then begin
    raise Exception.Create(format('dx error %d: %s, %s', [(e shr 16) and $7FFF, DXGetErrorString9(e),
      DXGetErrorDescription9(e)]));
    exit(false);
  end;
  result := true;
end;

and I have dx error 0: E_FAIL, An Undetermined error occured at

  CheckDxError(m_pd3dDevice.CreateVertexDeclaration(@decl, g_pVertexDeclaration));

DirectX 9, device etc initialized before call (and code works without shaders, I trying to study shaders), there was no error in this code with decl array with only first position and D3DDECL_END. But my vertex shader contains 4 input values and compiled without errors with fxc.exe from DirectX SDK.

struct VSInput
{
    float3 Pos : POSITION;
    float3 Normal : NORMAL;
    float2 TexcoordUV : TEXCOORD0;
    float2 TexcoordST : TEXCOORD1;
};

Why this error occurs and what to do to avoid?

Answer 1

Your second TEXCOORD needs to have UsageIndex set to 1.

The documentation for D3DDECLUSAGE does cover this:

D3DDECLUSAGE_TEXCOORD

Texture coordinate data. Use D3DDECLUSAGE_TEXCOORD, n to specify texture coordinates in fixed function vertex processing and in pixel shaders prior to ps_3_0. These can be used to pass user defined data.

The value of n is the usage index and if you want to have multiple vertex elements with the same usage, you need to give them distinct usage indices.