问题
how do i find out pixel value at certain degree on the circumference of a circle if I know the pixel co-ordinates of the center of the circle, radius of the circle ,and perpendicular angle. Basically, I am trying to draw the hands of a clock at various times ( 1 o clock , 2 o clock etc )
回答1:
Let h be the hour as a floating point number (h=2.25 would be 02:15, etc.) between 0 and 12. (cX,cY) are the coordinates of the center. hLength and mLength are the lengths of the hour and min hands.
// Hour hand
hAngle = 2.0*Pi*h/12.0; // 0..12 mapped to 0..2*Pi
hX = cX + hLength * sin(hAngle);
hY = cY - hLength * cos(hAngle);
// Min hand
mAngle = 2.0*Pi*h; // 0..1 mapped to 0..2*Pi, etc.
mX = cX + mLength * sin(mAngle);
mY = cY - mLength * cos(mAngle);
回答2:
Where the centre of the circle is (X0, Y0), the radius is R and the angle with the x-axis is theta:
X1 = (R * cos theta) + X0
and
Y1 = (R * sin theta) + Y0
回答3:
If (x1,y1) is a point on the circumference and (x,y) is the center, then x1 = x + r * cos(angle) and y1 = y + r * sin(angle)
回答4:
if center is at x0, y0, and 0,0 iz at bottom-left corner, then 1 o'clock is at x0 + rsin(2π/3), y0+rcos(2π/3).
回答5:
Draw lines from the center to coordinates computed with sin for the y coordinates and cos for the x coordinates (both multiplied by the length of the hand).
Wikipedia has more information on how sin and cos "work".
来源：https://stackoverflow.com/questions/925118/algorithm-for-finding-out-pixel-coordinates-on-a-circumference-of-a-circle