Okay, I think I found a good approximation of what I'm trying to do, though I'd still like to hear if anyone else has a better solution. By checking the tangent of the angle I've actually traveled so far between each pixel move, I can decide whether to move one pixel in the x-direction, or in the y-direction.
for i in range(len(angles)):
angle = self.orientation+angles[i]
if angle > 360:
angle -= 360
x = self.x
y = self.y
x1 = x
y1 = y
xtoy_ratio = tan(angle*math.pi/180)
if angle < 90:
xadd = 1
yadd = 1
elif 90 < angle < 180:
xadd = -1
yadd = 1
elif 180 < angle < 270:
xadd = -1
yadd = -1
else:
xadd = 1
yadd = -1
while mapp.getpixel(x,y) != (0,0,0,255):
if (y-y1)/(x-x1) < xtoy_ratio:
y += yadd
else:
x += xadd
distance = sqrt((y-y1)^2+(x-x1)^2)*pixel_width
The accuracy of this method of course depends a great deal on the actual length represented by each pixel. As long as pixel_width is small, accuracy will be pretty good, but if not, it will generally go pretty far before correcting itself.
As I said, I welcome other answers.
Thanks