Bitmap rotation

How do you rotate a bitmap image? Rotating each pixel doesn't work. There must be a way, because The GIMP can do it.

I'm using the SDL.

If you don't mind using a module you could use
http://freshmeat.net/projects/sge/

else read http://www.gamedev.net/reference/art...article811.asp
The important part of that article is the multiplication matrix it gives.

Thanks. I'll see if I can get it to work.

Do you mean rotating the actual texture on the surface it is applied to, or rotating the surface?

Rotate the quad that the texture uses.

What do you mean?

Think:
* What is the difference between a normal bitmap and a rotated one?
* What happens to the bounding rectangle of the bitmap when you rotate the bitmap?
* What is the difference between a rotated quadrilateral in 3D graphics and a bounding rectangle of a bitmap?


You can rotate a bitmap many ways. If you really want to use 2D and not 3D vertices in that you want to rotate a blitted bitmap you can do this:

1. Set texture coords for each corner of the box.
2. Compute the new bounding box after rotation.
3. Polyfill the box using linear interpolation.
4. Figure out the x,y coords of pixel.
5. Figure out the u,v coords of texel.

Essentially linear interpolate for x,y and u,v.

Pixel(x,y,Texture[u][v]);

To do a polyfill:

1. Create two arrays called leftside and rightside and clear to -1.
2. Start at the top of the figure.
3. Find the next vertex in either direction.
4. If vertex is lower than prev, it goes in leftside array
5. If vertex is higher than prev, it goes in rightside array.
6. Compute x's and u,v for leftside and rightside at point P on a line between CurVertex and NextVertex using formula for slope.
7. Save x's in leftside and rightside accordingly and increment a y line counter.
8. When done, start at y coord for top and set loopcounter to 0.
9. Draw textured line from leftside[loopcounter] to rightside[loopcounter]
10. Increment y and loop counter and check for loopcounter>y line counter.
11. If true, exit, else goto 9.

This is a simple old-school polygon fill algo used in many older DOS 3D games. It only works on convex shapes.

This will draw a bitmap w/o rotation and scaling.

Code:

---


DWORD dwTexOffset=0;
UINT uCurX=Texture.X;
UINT uCurY=Texture.Y;
DWORD dwPixelOffset=(uCurY*Screen.Width)+uCurX;
DWORD dwStartPixelOffset=dwPixelOffset;
DWORD dwWidthCounter=0;
DWORD dwHeightCounter=0;

do
{
  DWORD dwPixelColor=Texture[dwTexOffset];
  if (dwPixelColor!=Texture.dwTransparent)       
    SetPixel(dwPixelOffset,Texture[dwTexOffset];

  dwTexOffset++;
  dwPixelOffset++;
  dwWidthCounter++;
  if (dwWidthCounter>TextureWidth)
  {
    dwStartPixelOffset+=Screen.Width;
    dwPixelOffset=dwStartPixelOffset;
    dwWidthCounter=0;
    dwHeightCounter++;
  }
} while (dwHeightCounter<TextureHeight);

---

This code is best suited for assembly. You can use inline or pure. I'll write an inline version here.

Code:

---

void BitBltEx(WORD uX, WORD uY,
                    DWORD *pSurface,
                    DWORD *pTexture,
                    DWORD dwTexWidth,DWORD dwTexHeight,
                    DWORD dwSurfWidth,
                    DWORD dwTransColor)
{
  // (ESP-4)  DWORD dwTexOffset=0;
  // (ESP-8)  DWORD dwPixelOffset;
  // (ESP-12) DWORD dwStartPixelOffset;
  // (ESP-16) DWORD dwWidthCounter=0;
  //Height counter not needed since LOOP decrements ECX
  _asm {
         
      ;Allocate stack space for local variables
      sub esp,16

      ;dwTexOffset=0;
      mov [esp-4],0

      ;Compute surface offset
      mov  eax,uY
      mov  ebx,dwSurfWidth
      mul  eax,ebx
      add  eax,uX
      mov [esp-8],eax
      mov [esp-12],eax

      ;Zero out counters
      mov [esp-16],0

      ;Set up pointers
      mov esi,[pTexture]
      mov edi,[pSurface]
      add edi,[esp-8]

      ;Setup loop
      LOOPSTART:
      mov ecx,dwTexHeight

      ;Blit one pixel if cur pixel is not trans
      ;Retrieve current pixel
      lodsd
      cmp eax,dwTransColor
      jne  NOBLIT
      ;BLIT here
      movsd

      NOBLIT:
      //Increment counters
      //esi and edi auto-increment according to direction flag
      //on movsd
      inc [esp-16]

      cmp [esp-16],dwTexWidth
      jb  SAMELINE
      add [esp-12],dwSurfWidth
      mov eax,[esp-12]
      mov [esp-8],eax
      mov [esp-16],0
      SAMELINE:
      LOOP LOOPSTART
 
      ;clean up stack
      add esp,16
  }
}

---

The assembly version could be sped up but my asm is a bit rusty.
This should work if you can get a pointer to surface memory and texture memory. However, I'm sure SDL provides a much better way. This is really old-school 2D BitBlts here.

Use the asm at your own risk. I wrote it here while looking at the C version, so I'm sure it has some bugs. I did refer to the IA32 Tech Ref for some of the instructions. For the most part it should give you the general idea.

Neither of these functions accounts for clipping to a bounding area.