VCMFCGDI位图旋转算法.doc

VC MFC GDI 位图旋转算法 2010-04-30 19:39 Function 1 newx = x.cos(angle) + y.sin(angle) newy = y.cos(angle) - x.sin(angle) 是以位图左上角为原点逆时针旋转X弧度 newx = x.cos(angle) - y.sin(angle) newy = y.cos(angle) + x.sin(angle) 是以位图左上角为原点顺时针旋转X弧度 Here are the steps that we take to rotate the bitmap. Create a couple of device contexts compatible with the display. One of them will be used to hold the source bitmap and the other will hold the destination bitmap (for the rotated image). Precompute the cosine and the sine of the angle. This will help save a few microseconds in subsequent computations. Compute the bounding rectangle of the rotated image. We use the formula newx = x.cos(angle) + y.sin(angle) newy = y.cos(angle) - x.sin(angle) We assume one of the corners (0,0) to be the center of rotation and therefore need to calculate the new co-ordinates of the other three corners. Based on this we can determine the widht and height of the new bitmap. Since the rotated image will not occupy the entire area of the new bitmap, we fill the destination bitmap with the background color specified through the function argument. Since we will use NTs support for linear transformation we set the graphic mode of the destination DC to support this. We set up the XFORM struction and call SetWorldTransform() to activate the transformation. The SetWorldTransform() function sets up the stage for BitBlt(), which is when the transformation takes place. The algorithm used for the linear transformation is newx = x * eM11 + y * eM21 + eDx newy = x * eM12 + y * eM22 + eDy For rotation eM11 and eM22 should be the cosine of the rotation angle, eM12 should be the sine of the rotation angle and eM21 should be -eM12. In the DC we are using, since the +ve y direction is downwards, we reverse the signs of eM12 and eM21. We also set the translation components (eDx eDy) so that rotated image fits inside the new bitmap without part of it getting clipped. We finally call BitBlt() to do the actual rotation.