Sprite rotation offset doesn't stay where it belongs. (SDL) - c++

Question

So, here is the code for my 2D point class to rotate:
float nx = (x * cos(angle)) - (y * sin(angle));
float ny = (y * cos(angle)) + (x * sin(angle));
x = nx;
y = ny;
x and y are local variables in the point class.
And here is the code for my sprite class's rotation:
//Make clip
SDL_Rect clip;
clip.w = width;
clip.h = height;
clip.x = (width * _frameX) + (sep * (_frameX) + osX);
clip.y = (height * _frameY) + (sep * (_frameY) + osY);
//Make a rotated image
col bgColor = image->format->colorkey;
//Surfaces
img *toEdit = newImage(clip.w, clip.h);
img *toDraw = 0;
//Copy the source into the workspace
drawRect(0, 0, toEdit->w, toEdit->h, toEdit, bgColor);
drawImage(0, 0, image, toEdit, &clip);
//Edit the image
toDraw = SPG_Transform(toEdit, bgColor, angle, xScale, yScale, SPG_NONE);
SDL_SetColorKey(toDraw, SDL_SRCCOLORKEY, bgColor);
//Find new origin and offset by pivot
2DVec *pivot = new xyVec(pvX, pvY);
pivot->rotate(angle);
//Draw and remove the finished image
drawImage(_x - pivot->x - (toDraw->w / 2), _y - pivot->y - (toDraw->h / 2), toDraw, _destination);
//Delete stuff
deleteImage(toEdit);
delete pivot;
deleteImage(toDraw);
The code uses the center of the sprite as the origin. It works fine if I leave the pivot at (0,0), but if I move it somewhere else, the character's shoulder for instance, it starts making the sprite dance around as it spins like a spirograph, instead of the pivot staying on the character's shoulder.
The image rotation function is from SPriG, a library for drawing primitives and transformed images in SDL. Since the pivot is coming from the center of the image, I figure the new size of the clipped surface produced by rotating shouldn't matter.
[EDIT]
I've messed with the code a bit. By slowing it down, I found that for some reason, the vector is rotating 60 times faster than the image, even though I'm not multiplying anything by 60. So, I tried to just divide the input by 60, only now, it's coming out all jerky and not rotating to anything between multiples of 60.
The vector rotation code I found on this very site, and people have repeatedly confirmed that it works, so why does it only rotate in increments of 60?

Answer 1

I haven't touched the source of SPriG in a long time, but I can give you some info.
If SPriG has problems with rotating off of center, it would probably be faster and easier for you to migrate to SDL_gpu (and I suggest SDL 2.0). That way you get a similar API but the performance is much better (it uses the graphics card).
I can guess that the vector does not rotate 60 times faster than the image, but rather more like 57 times faster! This is because you are rotating the vector with sin() and cos(), which accept values in radians. The image is being rotated by an angle in degrees. The conversion factor for radians to degrees is 180/pi, which is about 57. SPriG can use either degrees or radians, but uses degrees by default. Use SPG_EnableRadians(1) to switch that behavior. Alternatively, you can stick to degree measure in your angle variable by multiplying the argument to sin() and cos() by pi/180.