Ellipses (that stretch) in DirectDraw

'ello everybody.

I searched for this topic (assumed someone had to have
asked it before -- which they did), but still had questions
about how to go about this. The math, etc..

So, I'm making a really simple 2D game. Right now I'd like to
add ellipses underneath the objects on the screen to simulate
shadows. These ellipses would shrink as the object gets higher.
You know, Super Mario Bros shadow effects. :P


btw, I'm very rusty at programming right now. I haven't really
done anything in a couple years...


Now, I know I can do this in GDI, but wouldn't that only
draw the ellipse to the Device Context of the Window
and not the DDraw Surface I'm using as a back buffer?
That would mean they wouldn't be in my screenshots...

Would locking the surface and coloring each pixel black
be the best way, then?

Now, here's my real problem: How do I make an ellipse
that stretches? I mean, math-wise. And if I am able to draw
the actual ellipse, how would I color it in?

i would set the original size of the shadow equals to the width of the characters/objects while they are on the ground. As the height is subtracted/added (depend on the screen perspective you set) from the object's coordinate, you decrease the wide of the ellipse.
Also, set the X axis of the ellipse relatively to the object's X axis, and the Y axis to the ground's Y axis.

I don't know how to solve your problem in DX but I do in D3D.

1. Turn alpha blending on in game.
2. Create a bitmap in a paint program of a 50% grey circle.
3. Create a mask for the bitmap which masks out the circle and save to disk. (Circle=white, other=black)
4. Fire up DX texture tool and load bitmap.
5. Change surface format to A8R8G8B8.
6. Select File->Open onto alpha channel of this texture
7. Select your mask bitmap.
8. Use blend mode: D3DRS_SRCBLEND,D3DBLEND_SRCALPHA
9. Use blend mode: D3DRS_DESTBLEND,D3DBLEND_INVSRCALPHA
10. Scale the quad to suit sprite size.
11. Render the quad using the resulting texture.

Code:

#ifndef TLVERTEX
#define TLVERTEX

#include <d3dx9h>

struct TLVertex
{
  //Position
  D3DXVECTOR3 Pos;
  //Homogenous W - 1.0f
  float RHW;

  //Diffuse color
  D3DCOLOR dwDiffuse;

  //Texture coords
  float u,v;

  //Flexible Vertex Format
  static const DWORD FVF;

  //Various constructors
  TLVertex(void):Pos(D3DXVECTOR3(0.0f,0.0f,0.0f)),RHW(1.0f),dwDiffuse(0),u(0.0f),v(0.0f) {}
  TLVertex(D3DXVECTOR3 _Pos,float _u,float _v,D3DCOLOR _dwDiffuse=0):Pos(_Pos),u(_u),v(_v),dwDiffuse(_dwDiffuse) {}

  TLVertex(float x,float y,float z,float _u,float _v,D3DCOLOR _dwDiffuse=0):D3DXVECTOR3(x,y,z),u(_u),v(_v),dwDiffuse(_dwDiffuse) {}

  //Assignment operator
  TLVertex &operator=(const TLVertex &vert) 
  {
    Pos=vert.Pos;
    RHW=vert.RHW;
    dwDiffuse=vert.dwDiffuse;
    u=vert.u;
    v=vert.v;
    FVF=vert.FVF;
    return *this;
  }
};
#endif

Code:

#include "CObject.h"
#include "TLVertex.h"

//Define FVF for TLVertex
const DWORD TLVertex::FVF=D3DFVF_XYZRHW | D3DFVF_DIFFUSE | D3DFVF_TEX1;

void CObject::Create(IDirect3DDevice9 *pDevice,D3DXVECTOR3 Pos,float fXSize,float fYSize)
{
  //Init class vars
  m_pDevice=pDevice;
  m_fXSize=fXSize;
  m_fYSize=fYSize;
  m_vPos=Pos;
  
  //Create vertices
  m_pVerts[0]=TLVertex(0.0f,0.0f,1.0f,0.0f,0.0f);
  m_pVerts[1]=TLVertex(1.0f,0.0f,1.0f,1.0f,0.0f);
  m_pVerts[2]=TLVertex(0.0f,1.0f,1.0f,0.0f,1.0f);
  m_pVerts[3]=TLVertex(1.0f,1.0f,1.0f,1.0f,1.0f);
}

void CObject::LoadTexture(std::string File)
{
  //Load and create texture
  D3DXCreateTextureFromFile(m_pDevice,File.c_str(),&m_pTexture);
}

void CObject::PreRender(float fTimeDelta)
{
  //Set source and dest blends
  m_pDevice->SetRenderState(D3DRS_SRCBLEND,D3DBLEND_SRCALPHA);
  m_pDevice->SetRenderState(D3DRS_DESTBLEND,D3DBLEND_INVSRCALPHA);
}

void CObject::Render(float fTimeDelta)
{
  //Setup scaling matrix
  D3DXMATRIX matScale;
  D3DXMatrixScaling(&matScale,m_fSize.x,m_fSize.y,0.0f);

  //Setup translation matrix
  D3DXMATRIX matTrans;
  D3DXMatrixTranslation(&matTrans,m_vPos.x,m_vPos.y,1.0f);
    
  TLVertex Verts[4];

  for (int i=0;i<4;i++)
  {
     //Copy source verts
     Verts[i]=m_pVerts[i];

     //Transform verts - scale and then translate
     D3DXVec3TransformCoord(&Verts[i],&matScale,m_pVerts[i]);
     Verts[i]*=matTrans;
  }

  //Set FVF
  m_pDevice->SetFVF(TLVertex::FVF);

  //Set Texture
  m_pDevice->SetTexture(m_pTexture);

  //Draw
  m_pDevice->DrawPrimitive(D3PT_TRIANGLESTRIP,2,&Verts,sizeof(TLVERTEX));
}