Opengl 2D Texture Rotation - Simples. Or maybe not.

[code_glitch]

So I was working on my engine a bit and unfortunately made a call to Image.Rotate(Deg: Int64) and its not that it does not work, it just does not work as you would expect it to...

To rotate the texture I am using:

procedure Image.Rotate(Deg: Int64);
begin
    glPushAttrib(GL_TRANSFORM_BIT);
    glMatrixMode(GL_TEXTURE);
    glLoadIdentity();
    glBindTexture( GL_TEXTURE_2D, texture);
    glTranslatef(0.5, 0.5, 0);
    glRotatef(Deg, 0, 0, 1);
    glTranslatef(-0.5, -0.5, 0);
    glPopAttrib();
end;

Which as far as I am concerned is pretty simple and straight forward, and drawing also follows suit with:

procedure Image.Draw(X,Y: Int64);
var
    VxR: array [1..2] of LongInt;
    VyR: array [1..2] of LongInt;
    
    PxR: array [1..2] of Int64;
    PyR: array [1..2] of Int64;
    
begin
    VxR[1] := 0;
    VxR[2] := XScale;
    VyR[1] := 0;
    VyR[2] := YScale;
    
    PxR[1] := X;
    PxR[2] := X + GlWidth;
    PyR[1] := Y;
    PyR[2] := Y + GlHeight;

    glBindTexture( GL_TEXTURE_2D, texture );

    glBegin( GL_QUADS );
    glTexCoord2i( VxR[1], VyR[2] );
    glVertex3f( PxR[1], PyR[2], 0. );

    glTexCoord2i( VxR[2], VyR[2] );
    glVertex3f( PxR[2], PyR[2], 0. );

    glTexCoord2i( VxR[2], VyR[1] );
    glVertex3f( PxR[2], PyR[1], 0. );

    glTexCoord2i( VxR[1], VyR[1] );
    glVertex3f( PxR[1], PyR[1], 0. );
    glEnd();
end;

(please don't ask about variable names. This is really old code from me )

and I get this 'interesting' result:
Rotation Glitch.jpg
(yes, its an alchemy logo because its the only small png file I had off hand XD )

Is there any way to get the new X, Y, Height and Width co-ordinates without drawing that garbage data round the image?

cheers,
code_glitch.

ps: sorry for the long post.

[Andru]

On this small jpeg I don't see any garbage...

[code_glitch]

Well, it draws a square, and not the entire image is drawn. The rotated surface is like a diamond cropped at all corners by the rectangular draw. (not really good description)... and in the bottom left hand corner is the writing again, which is not from the original image (its like its tiled there?) here is the original:
alchemy.png

[Andru]

here is the original:

Oh, now I see what is the problem. Your problem is using glMatrixMode(GL_TEXTURE). You are rotates texture inside a quad, not the quad itself Or you just want to rotate texture inside quad, but "cut" it to something like this?

[code_glitch]

yes, thats it perfectly. I'm not used to reotating quads and stuff so I didn't know how to describe it... Anyhow, how would you rotate a quad? I'm sort of lost here unless you change glMatrixMode(GL_TEXTURE) to glMatrixMode(GL_QUAD)? and if so would you still not need to rotate the texture?

besides that, how would you draw it?

[Andru]

besides that, how would you draw it?

For batch optimizations and so on I don't use matrices for rotating. I'm using coordinates of sprite and rotates them by simple math Here the code from ZenGL:

  var
    x1, x2 : Single;
    y1, y2 : Single;
    cX, cY : Single;
    c, s   : Single;
// ..
    x1 := -Width / 2;
    y1 := -Height / 2;
    x2 := -x1;
    y2 := -y1;
    cX :=  X + x2;
    cY :=  Y + y2;

    m_SinCos( Angle * deg2rad, s, c );

    // Getting new coordinates for each vertex of rotated sprite. "p" is just a pointer to record with parameters X and Y, "quad" - array with 4 elements.
    p := @quad[ 0 ];
    p.X := x1 * c - y1 * s + cX;
    p.Y := x1 * s + y1 * c + cY;
    INC( p );
    p.X := x2 * c - y1 * s + cX;
    p.Y := x2 * s + y1 * c + cY;
    INC( p );
    p.X := x2 * c - y2 * s + cX;
    p.Y := x2 * s + y2 * c + cY;
    INC( p );
    p.X := x1 * c - y2 * s + cX;
    p.Y := x1 * s + y2 * c + cY;

About rotating as I show in attached image(texture inside quad) - I think you must enable GL_CLAMP for texture, but I don't sure if it will help in that situation.

[code_glitch]

Ok, woops. all I can say is GWAAAH!

It does indeed work BUT: houston we have a problem. here is what I mean: close but sort of impractical
Rotation Glitch MK 2.jpg
here is the code for the Test program:

    Im.Load('Img.png');
    //Im.Draw(100, 100);
    c := 0;
    repeat
        c := c + 1;
        Im.Rotate(c);
        Im.Draw(100,100);
        
        UpdateCanvas();
        delay(10);
        until c >= 360;

where I have changed rotate to use Gl_Quads instead of Gl_Texture...

edit: sorry did not refresh before posting...

[code_glitch]

I would like to use glGetTexParameteriv to find its center but returns an untyped variable... How do I load this into an Int64 say?

PS: Is it just me or does ZenGl do everything?

[Andru]

I would like to use glGetTexParameteriv

Errm... One more time about what do you want to do You want just rotate a sprite, or do rotation inside quad(as I showed in attached image of my second answer)?

[code_glitch]

rotate quad and texture around (working) the origin which would depend on its width, which as far as I know can be read from glGetTexParameteriv but this returns an untyped variable which I have no idea on how to get into a longint...