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I have a function to perform sprite rotation :

void Sprite::rotateSprite(float angle){
    //Making an array of vertices - 6 for 2 triangles

    Vector2<gamePos> halfDims(_rect.w / 2, _rect.h /2);

    Vector2<gamePos> bl(-halfDims.x,-halfDims.y);
    Vector2<gamePos> tl(-halfDims.x,halfDims.y);
    Vector2<gamePos> br(halfDims.x,-halfDims.y);
    Vector2<gamePos> tr(halfDims.x,halfDims.y);


    bl = rotatePoint(bl,angle) + halfDims;
    br = rotatePoint(br,angle) + halfDims;
    tl = rotatePoint(tl,angle) + halfDims;
    tr = rotatePoint(tr,angle) + halfDims;

    /**
        1st triangle
    **/

    //Top right
    _dataPointer.vertices[0].setPosition(_rect.x+tr.x,_rect.y+tr.y);

    //Top left
    _dataPointer.vertices[1].setPosition(_rect.x+tl.x,_rect.y+tl.y);

    //Bottom left
    _dataPointer.vertices[2].setPosition(_rect.x+bl.x,_rect.y+bl.y);

    /**
        2nd triangle
    **/

    //Bottom left
    _dataPointer.vertices[3].setPosition(_rect.x+bl.x,_rect.y+bl.y);

    //Bottom right
    _dataPointer.vertices[4].setPosition(_rect.x+br.x,_rect.y+br.y);

    //Top right
    _dataPointer.vertices[5].setPosition(_rect.x+tr.x,_rect.y+tr.y);
}

Vector2<gamePos> Sprite::rotatePoint(Vector2<gamePos> pos, float& angle){

    Vector2<gamePos> newv;

    newv.x = pos.x * cos(angle) - pos.y * sin(angle);
    newv.y = pos.y * cos(angle) + pos.x * sin(angle);

    return newv;
}

And the result is Image of problem

Am i doing something wrong ? It happens also when i put small angle (even if i put here angle 1 )

Thanks for help.

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  • \$\begingroup\$ Are you sure the angle is supposed to be in degrees and not radians? \$\endgroup\$ Feb 22, 2017 at 20:39
  • \$\begingroup\$ Man, you should really use shaders and matrices to rotate stuff around. \$\endgroup\$
    – Bálint
    Feb 22, 2017 at 20:58
  • \$\begingroup\$ @Bálint Yes I know, but I wanted to try also this alternative. \$\endgroup\$
    – Pins
    Feb 22, 2017 at 22:26
  • \$\begingroup\$ @Pins this requires you to constantly load in new vertices to the gpu, and that's slow \$\endgroup\$
    – Bálint
    Feb 22, 2017 at 23:01
  • \$\begingroup\$ @Bálint I thought that the thing with vertices is same, because I put every loop objects to spriteBatch(save vbo info + textureID) and buffer the data to GPU. Isn´t it right? \$\endgroup\$
    – Pins
    Feb 23, 2017 at 18:02

1 Answer 1

3
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Try

angle = angle *(3.14159/180.0)
newv.x = pos.x * cos(angle) - pos.y * sin(angle);
newv.y = pos.y * cos(angle) + pos.x * sin(angle);

Trig functions works with radians so you will have to do the conversion to radians as above. as stated in the comments, It is recommended you convert the (3.14159/180.0) to a constant and use the built in PI constant instead of 3.14159.

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4
  • 5
    \$\begingroup\$ OpenGL doesn't work with anything, the trigonometric functions need radians \$\endgroup\$
    – Bálint
    Feb 22, 2017 at 20:57
  • 3
    \$\begingroup\$ And try using built-in constants instead of 3.14159. Those usually go up to 20 decimal places. \$\endgroup\$
    – Bálint
    Feb 22, 2017 at 21:09
  • 3
    \$\begingroup\$ @Bálint For the sake of argument - OpenGL does work with something. Old OpenGL functions, alike glRotatef worked with angles in degrees. Modern OpenGL shaders trig functions work with radians. \$\endgroup\$
    – Kromster
    Feb 23, 2017 at 13:20
  • \$\begingroup\$ @Kromster If you count the deprecated stuff, then yes. \$\endgroup\$
    – Bálint
    Feb 23, 2017 at 19:05

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