 # Question on LineCollection

Hi,

I’m plotting thousands of short lines on a plot. Because “plot” and “Line2D” are quite slow for this case, I’m trying to use lineCollection. Here comes the part of my testing code:

``````...
segs = []

# Manual set for testing
x2 = np.zeros(2,dtype=int)
ys2 = [np.zeros(2,dtype=float)]
x2 = 1000000
x2 = 2000000
ys2 = ys2 = 50
segs.extend( [zip(x2,y) for y in ys2] )

for i in range(0, len(records)):
...
# get xStart, xEnd, and y
...

x2 = xStart
x2 = xEnd
ys2 = ys2 = y
segs.extend( [zip(x2,y) for y in ys2] )

line_segments = LineCollection(segs, linewidth=4, alpha=0.3, colors = 'r', linestyle = 'solid')
...
``````

The problem I have is only the first segment of the lines in the “segs” which I manually set the values is shown in the plot. If I move the first part in the for loop to test, it works. What am I missing?

Jin

···

2011/7/13 SULSEUNG-JIN <sulsj0270@...32...>:

Hi,

I'm plotting thousands of short lines on a plot. Because "plot" and "Line2D"
are quite slow for this case, I'm trying to use lineCollection. Here comes
the part of my testing code:

``````\.\.\.
segs = \[\]

\# Manual set for testing
x2 = np\.zeros\(2,dtype=int\)
ys2 = \[np\.zeros\(2,dtype=float\)\]
x2\[0\] = 1000000
x2\[1\] = 2000000
ys2\[0\]\[0\] = ys2\[0\]\[1\] = 50
segs\.extend\( \[zip\(x2,y\) for y in ys2\] \)

for i in range\(0, len\(records\)\):
\.\.\.
\# get xStart, xEnd, and y
\.\.\.

x2\[0\] = xStart
x2\[1\] = xEnd
ys2\[0\]\[0\] = ys2\[0\]\[1\] = y
segs\.extend\( \[zip\(x2,y\) for y in ys2\] \)

line\_segments = LineCollection\(segs, linewidth=4, alpha=0\.3, colors =
``````

'r', linestyle = 'solid')
...

The problem I have is only the first segment of the lines in the "segs"
which I manually set the values is shown in the plot. If I move the first
part in the for loop to test, it works. What am I missing?

Are you sure it's the first segment that shows up, and not the last one?

You're reusing the numpy array x2 and the list ys2. You're overwriting
the values inside them each time through the for loop, but segs just
contains len(records) references to the exact same arrays in memory
(x2 and ys2).

The reason it works if you move the first part inside is that it
creates a new numpy.array x2 and a new python list ys2 each time then,
which is what you want.

Justin

Thanks, Justin

I think I made a confusing example code. Here comes new one:

segs = []

for i in range(0, len(vec)):

``````x1 = vec[i]

#x1 = 3000000

x2 = vec[i]

#x2 = 4000000

y1 = y2 = vec[i]

segs.extend( [[(x1,y1),(x2,y2)]] )
``````

line_segments = LineCollection(segs, linewidth=3, alpha=0.3, colors = ‘r’, linestyle = ‘solid’) ax.add_collection(line_segments)

The above code doesn’t show any lines. But if I comment out one or both of commented parts, the lines are shown.

I mean the below cases are all work.

``````x1 = vec[i]

x2 = 4000000

y1 = y2 = vec[i]
``````

or

``````x1 = 3000000

x2 = vec[i]

y1 = y2 = vec[i]
``````

I think it’s wierd. I’m pretty sure that the 2D array has valid values.

Jin

···

Date: Wed, 13 Jul 2011 15:32:33 -0400
Subject: Re: [Matplotlib-users] Question on LineCollection
From: jneilm@…287…
To: sulsj0270@…32…
CC: matplotlib-users@lists.sourceforge.net

2011/7/13 SULSEUNG-JIN <sulsj0270@…32…>:

Hi,

I’m plotting thousands of short lines on a plot. Because “plot” and “Line2D”
are quite slow for this case, I’m trying to use lineCollection. Here comes
the part of my testing code:

``````...
segs = []

# Manual set for testing
x2 = np.zeros(2,dtype=int)
ys2 = [np.zeros(2,dtype=float)]
x2 = 1000000
x2 = 2000000
ys2 = ys2 = 50
segs.extend( [zip(x2,y) for y in ys2] )

for i in range(0, len(records)):
...
# get xStart, xEnd, and y
...

x2 = xStart
x2 = xEnd
ys2 = ys2 = y
segs.extend( [zip(x2,y) for y in ys2] )

line_segments = LineCollection(segs, linewidth=4, alpha=0.3, colors =
``````

‘r’, linestyle = ‘solid’)

The problem I have is only the first segment of the lines in the “segs”
which I manually set the values is shown in the plot. If I move the first
part in the for loop to test, it works. What am I missing?

Are you sure it’s the first segment that shows up, and not the last one?

You’re reusing the numpy array x2 and the list ys2. You’re overwriting
the values inside them each time through the for loop, but segs just
contains len(records) references to the exact same arrays in memory
(x2 and ys2).

The reason it works if you move the first part inside is that it
creates a new numpy.array x2 and a new python list ys2 each time then,
which is what you want.

Justin

2011/7/13 SULSEUNG-JIN <sulsj0270@...32...>:

Thanks, Justin

I think I made a confusing example code. Here comes new one:

Maybe you just need to force a call to draw() and set your x/y limits.
This works for me on matplotlib 1.0.1

\$ ipython -pylab
# ====
from matplotlib.collections import LineCollection
f = figure()
plot()
ax = gca()
vec = numpy.random.random((10,3))
segs = []
for i in range(0, len(vec)):
x1 = vec[i]
#x1 = 3000000
x2 = vec[i]
#x2 = 4000000
y1 = y2 = vec[i]
segs.extend( [[(x1,y1),(x2,y2)]] )

line_segments = LineCollection(segs, linewidth=3, alpha=0.3, colors =
'r', linestyle = 'solid')
ax.set_xlim(0,1)
ax.set_ylim(0,1)
show() # hmmmm... nothing yet
draw() # force a draw; now it works

Justin, your code works flawlessly. Well, I’ve got the “vec” from pytables like:

vec = [ [x[‘sStart’],x[‘sEnd’],x[‘dIdent’],x[‘gi’] ] for x in table.where(’(gi == currGi) & (sId == currSubId)’) ]

With the above “vec”, it does not work. Do you have any idea?

Jin.

···

Date: Wed, 13 Jul 2011 16:44:01 -0400
Subject: Re: [Matplotlib-users] Question on LineCollection
From: jneilm@…287…
To: sulsj0270@…32…
CC: matplotlib-users@lists.sourceforge.net

2011/7/13 SULSEUNG-JIN <sulsj0270@…32…>:

Thanks, Justin

I think I made a confusing example code. Here comes new one:

Maybe you just need to force a call to draw() and set your x/y limits.
This works for me on matplotlib 1.0.1

\$ ipython -pylab

# ====

from matplotlib.collections import LineCollection
f = figure()
plot()
ax = gca()
vec = numpy.random.random((10,3))
segs = []
for i in range(0, len(vec)):
x1 = vec[i]
#x1 = 3000000
x2 = vec[i]
#x2 = 4000000
y1 = y2 = vec[i]
segs.extend( [[(x1,y1),(x2,y2)]] )

line_segments = LineCollection(segs, linewidth=3, alpha=0.3, colors =
‘r’, linestyle = ‘solid’)
ax.set_xlim(0,1)
ax.set_ylim(0,1)
show() # hmmmm… nothing yet
draw() # force a draw; now it works

If I set like

vec = 100*numpy.random.random((10,3))

it does not shpw lines even with changing the set_xlimit and set_ylimit. I guess there is something related with scaling.

Jin

···

Date: Wed, 13 Jul 2011 16:44:01 -0400
Subject: Re: [Matplotlib-users] Question on LineCollection
From: jneilm@…287…
To: sulsj0270@…32…
CC: matplotlib-users@lists.sourceforge.net

2011/7/13 SULSEUNG-JIN <sulsj0270@…32…>:

Thanks, Justin

I think I made a confusing example code. Here comes new one:

Maybe you just need to force a call to draw() and set your x/y limits.
This works for me on matplotlib 1.0.1

\$ ipython -pylab

# ====

from matplotlib.collections import LineCollection
f = figure()
plot()
ax = gca()
vec = numpy.random.random((10,3))
segs = []
for i in range(0, len(vec)):
x1 = vec[i]
#x1 = 3000000
x2 = vec[i]
#x2 = 4000000
y1 = y2 = vec[i]
segs.extend( [[(x1,y1),(x2,y2)]] )

line_segments = LineCollection(segs, linewidth=3, alpha=0.3, colors =
‘r’, linestyle = ‘solid’)
ax.set_xlim(0,1)
ax.set_ylim(0,1)
show() # hmmmm… nothing yet
draw() # force a draw; now it works

Just an observation (I haven't tested anything)... But what is up with
the call to plot()? It might be causing issues with the autoscaler.
Any line collections created without other plotting functions is going
to need the axes limits set.

Ben Root

···

On Wednesday, July 13, 2011, Justin McCann <jneilm@...287...> wrote:

2011/7/13 SULSEUNG-JIN <sulsj0270@...32...>:

Thanks, Justin

I think I made a confusing example code. Here comes new one:

Maybe you just need to force a call to draw() and set your x/y limits.
This works for me on matplotlib 1.0.1

\$ ipython -pylab
# ====
from matplotlib.collections import LineCollection
f = figure()
plot()
ax = gca()
vec = numpy.random.random((10,3))
segs = []
for i in range(0, len(vec)):
x1 = vec[i]
#x1 = 3000000
x2 = vec[i]
#x2 = 4000000
y1 = y2 = vec[i]
segs.extend( [[(x1,y1),(x2,y2)]] )

line_segments = LineCollection(segs, linewidth=3, alpha=0.3, colors =
'r', linestyle = 'solid')
ax.set_xlim(0,1)
ax.set_ylim(0,1)
show() # hmmmm... nothing yet
draw() # force a draw; now it works

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\$ ipython -pylab
# ====
from matplotlib.collections import LineCollection
f = figure()
plot()
ax = gca()
vec = numpy.random.random((10,3))
segs = []
for i in range(0, len(vec)):
x1 = vec[i]
#x1 = 3000000
x2 = vec[i]
#x2 = 4000000
y1 = y2 = vec[i]
segs.extend( [[(x1,y1),(x2,y2)]] )

line_segments = LineCollection(segs, linewidth=3, alpha=0.3, colors =
'r', linestyle = 'solid')
ax.set_xlim(0,1)
ax.set_ylim(0,1)
show() # hmmmm... nothing yet
draw() # force a draw; now it works

...

Just an observation (I haven't tested anything)... But what is up with
the call to plot()? It might be causing issues with the autoscaler.
Any line collections created without other plotting functions is going
to need the axes limits set.

Ben Root

Yeah, that was weird. I added that when I was first messing around, and the axes didn't show
up even when I called show(). If you do the draw() at the end, then
you don't need to call plot(), and then you also don't need to mess
with the view limits (set_{x,y}lim).

I guess the moral of the story is, "if you don't explicitly plot() [or
a variant], you must explicitly draw()."

···

On Wed, Jul 13, 2011 at 6:49 PM, Benjamin Root <ben.root@...1304...> wrote:

On Wednesday, July 13, 2011, Justin McCann <jneilm@...287...> wrote:

===
from matplotlib.collections import LineCollection
f = figure()
ax = gca()
vec = numpy.random.random((10,3))
segs = []
for i in range(0, len(vec)):
x1 = vec[i]
x2 = vec[i]
y1 = y2 = vec[i]
segs.extend( [[(x1,y1),(x2,y2)]] )