velocity conversion

[Jerry]

I am moving an object using the vizact.move command which takes
meters/sec as an argument for speed.
How would I convert the meters/sec velocity to a screenpixels/sec velocity?

[farshizzo]

If the object is added to the screen (viz.ORTHO), then the units will already by in pixels/sec. If the object is added to the world (viz.WORLD), then it gets more complicated. Is your object part of the screen or world?

[Jerry]

The object is in the world. I know it's more complicated - that's
why I know you have the answer.

[farshizzo]

I'm assuming that the viewpoint is fixed while the object is moving? Otherwise it will be REALLY complicated ;)

[Jerry]

Yes, the viewpoint is stationary.

[farshizzo]

Here is a script that creates an action that is similar to vizact.goto() except the speed is in pixels/sec. Run the script and press space bar to run the action.import viz
viz.go()

class MovePixelSpeedAction(viz.ActionClass):

def convertToPixels(self,pos):
screen = self.window.worldToScreen(pos)
screen[0] *= self.size[0]
screen[1] *= self.size[1]
screen[2] = 0.0
return screen

def intersectLine(self,line1,line2):
o1 = viz.Vector(line1.getBegin())
o2 = viz.Vector(line2.getBegin())
d1 = viz.Vector(line1.getDir(),normalize=True)
d2 = viz.Vector(line2.getDir(),normalize=True)
c = d1 ^ d2
clen = c.length()
if clen == 0.0:
return o1

#Compute determinant
v = o2 - o1
d = (v[0] * (d2[1]*c[2] - d2[2]*c[1])) - (v[1] * (d2[0]*c[2]-d2[2]*c[0])) + (v[2] * (d2[0]*c[1]-d2[1]*c[0]))

d1.setLength(d / (clen * clen))
return o1 + d1

def begin(self,object):

self.window = viz.MainWindow
self.size = self.window.getSize(viz.WINDOW_PIXELS)

#Get begin/end positions
beginVal = object.getPosition()
endVal = vizact._paramlist_(self._actiondata_.pos,object)

#Convert to pixel coordinates
beginScreenVal = self.convertToPixels(beginVal)
endScreenVal = self.convertToPixels(endVal)

#Compute distance to travel
dist = vizmat.Distance(beginScreenVal,endScreenVal)
if dist == 0.0:
self.end(object)
return

#Get velocity
vel = vizact._paramval_(self._actiondata_.vel,object)

#Calculate duration
self.duration = dist / vel

#Initialize variables
self.elapsed = 0.0
self.endVal = endVal
self.beginScreenVal = beginScreenVal
self.endScreenVal = endScreenVal
self.worldLine = viz.Line(begin=beginVal,end=endVal)

def update(self,elapsed,object):

self.elapsed += elapsed
p = self.elapsed / self.duration
if p >= 1.0:
self._overtime_ = self.elapsed - self.duration
object.translate(self.endVal)
self.end(object)
return

#Interpolate screen pos from screen velocity
screenPos = vizmat.Interpolate(self.beginScreenVal,self.endScr eenVal,p)

#Convert screen location to world vector
line = self.window.screenToWorld( [screenPos[0]/self.size[0],screenPos[1]/self.size[1]] )

#Intersect vector with world vector of movement path
pos = self.intersectLine(self.worldLine,line)
object.translate(pos)

def movePixelSpeed(pos,vel):
bla = viz.ActionData()
bla.pos = pos
bla.vel = vel
bla.actionclass = MovePixelSpeedAction
return bla

START_POS = (0,0,8)
STOP_POS = (0,4,30)

ball = viz.add('ball.wrl',pos=START_POS)

def RunAction():
ball.translate(START_POS)
ball.runAction(movePixelSpeed(STOP_POS,100))

vizact.onkeydown(' ',RunAction)

[Jerry]

We are all humbled by your brilliance (I mean it!)