Filled contour transparency issue

Community matplotlib-users
1

Hello,

I am attempting to overlay a filled contour over a custom image.

I have managed to get something basic working, but i have encountered a
problem:

When the contourf plot is set to semi-transparent there are visible lines
joining the bottom of the plot and the filled contour edges. I have attached
an image of the plot.

http://img232.imageshack.us/my.php?image=spambs6.png spam.png

The code i have used is below; please bear in mind I am not a programmer, so
if the code seems botched, that's because it is.

···

###############################

import matplotlib.pyplot as plt
from pylab import *
try:
    import Image
except ImportError, exc:
    raise SystemExit("PIL must be installed to run this example")

lena = Image.open('lena.jpg')
dpi = rcParams['figure.dpi']
figsize = lena.size[0]/dpi, lena.size[1]/dpi

fig = plt.figure(figsize=figsize)
#fig.patch.set_alpha(0.5)
ax = fig.add_subplot(111)
#ax.patch.set_alpha(0.5)

ax.imshow(lena, origin='lower')

ax.contourf(z2,[10,15,20,25,30,35,40,45,50,55,60,65],alpha=0.7)
show()
############################

Can anyone help me with this problem?

Thanks for reading,
Pete.
--
View this message in context: http://www.nabble.com/Filled-contour-transparency-issue-tp18850187p18850187.html
Sent from the matplotlib - users mailing list archive at Nabble.com.

2

This is a known issue with the contouring code. It's borrowed from an earlier plotting package called GIST, and assumes that the renderer can not handle compound polygons (for example, donut-shaped, with both an inner and outer edge). So instead, it draws "cuts" that go from the inner to the outer edge. When anti-aliasing is turned on, there is a slight overlap or "drawing twice" along this edge which creates what looks like a line. Unfortunately, this can't be remedied by plotting the contour fills and then plotting the contour edges over top (as is done is contourf_demo.py).

As of matplotlib 0.98.x, matplotlib itself can handle compound paths, so we no longer need the cuts. I've made a few attempts at updating the contouring code to avoid them, but got nowhere. The contouring code is very opaque, almost magical code, and most who've dared to go in have barely made it out alive... :wink: That said, a fresh pair of eyes may have what it takes...

As for a workaround, you could render your contour as opaque, save that out as an image and read it back in. The code to do that won't be pretty, but it just might work.

Cheers,
Mike

kippertoffee wrote:

···

Hello,

I am attempting to overlay a filled contour over a custom image.

I have managed to get something basic working, but i have encountered a
problem:

When the contourf plot is set to semi-transparent there are visible lines
joining the bottom of the plot and the filled contour edges. I have attached
an image of the plot.

ImageShack - Best place for all of your image hosting and image sharing needs spam.png

The code i have used is below; please bear in mind I am not a programmer, so
if the code seems botched, that's because it is.

###############################

import matplotlib.pyplot as plt
from pylab import *
try:
    import Image
except ImportError, exc:
    raise SystemExit("PIL must be installed to run this example")

lena = Image.open('lena.jpg')
dpi = rcParams['figure.dpi']
figsize = lena.size[0]/dpi, lena.size[1]/dpi

fig = plt.figure(figsize=figsize)
#fig.patch.set_alpha(0.5)
ax = fig.add_subplot(111)
#ax.patch.set_alpha(0.5)

ax.imshow(lena, origin='lower')

ax.contourf(z2,[10,15,20,25,30,35,40,45,50,55,60,65],alpha=0.7)
show()
############################

Can anyone help me with this problem?

Thanks for reading,
Pete.
  
--
Michael Droettboom
Science Software Branch
Operations and Engineering Division
Space Telescope Science Institute
Operated by AURA for NASA

3

Thank you for your reply Michael.

Is it possible to make the lines in the contourf plot completely transparent
using an alpha setting? That would make a reasonable workaround as I intend
to overlay black contours anyway.

I've had a look at the source but it is way beyond me; I am a mere dabbler.

Cheers,
Pete.

Michael Droettboom-3 wrote:

···

This is a known issue with the contouring code. It's borrowed from an
earlier plotting package called GIST, and assumes that the renderer can
not handle compound polygons (for example, donut-shaped, with both an
inner and outer edge). So instead, it draws "cuts" that go from the
inner to the outer edge. When anti-aliasing is turned on, there is a
slight overlap or "drawing twice" along this edge which creates what
looks like a line. Unfortunately, this can't be remedied by plotting
the contour fills and then plotting the contour edges over top (as is
done is contourf_demo.py).

As of matplotlib 0.98.x, matplotlib itself can handle compound paths, so
we no longer need the cuts. I've made a few attempts at updating the
contouring code to avoid them, but got nowhere. The contouring code is
very opaque, almost magical code, and most who've dared to go in have
barely made it out alive... :wink: That said, a fresh pair of eyes may have
what it takes...

As for a workaround, you could render your contour as opaque, save that
out as an image and read it back in. The code to do that won't be
pretty, but it just might work.

Cheers,
Mike

kippertoffee wrote:

Hello,

I am attempting to overlay a filled contour over a custom image.

I have managed to get something basic working, but i have encountered a
problem:

When the contourf plot is set to semi-transparent there are visible lines
joining the bottom of the plot and the filled contour edges. I have
attached
an image of the plot.

http://img232.imageshack.us/my.php?image=spambs6.png spam.png

The code i have used is below; please bear in mind I am not a programmer,
so
if the code seems botched, that's because it is.

###############################

import matplotlib.pyplot as plt
from pylab import *
try:
    import Image
except ImportError, exc:
    raise SystemExit("PIL must be installed to run this example")

lena = Image.open('lena.jpg')
dpi = rcParams['figure.dpi']
figsize = lena.size[0]/dpi, lena.size[1]/dpi

fig = plt.figure(figsize=figsize)
#fig.patch.set_alpha(0.5)
ax = fig.add_subplot(111)
#ax.patch.set_alpha(0.5)

ax.imshow(lena, origin='lower')

ax.contourf(z2,[10,15,20,25,30,35,40,45,50,55,60,65],alpha=0.7)
show()
############################

Can anyone help me with this problem?

Thanks for reading,
Pete.

--
View this message in context: http://www.nabble.com/Filled-contour-transparency-issue-tp18850187p18856313.html
Sent from the matplotlib - users mailing list archive at Nabble.com.

4

It doesn't seem to matter whether the filled contours are drawn with edges or not -- the cuts are still visible because they're caused by over-drawing of the fill. (You can set linewidth=0 to try this.) But maybe I misunderstand your question.

Cheers,
Mike

kippertoffee wrote:

···

Thank you for your reply Michael.

Is it possible to make the lines in the contourf plot completely transparent
using an alpha setting? That would make a reasonable workaround as I intend
to overlay black contours anyway.

I've had a look at the source but it is way beyond me; I am a mere dabbler.

Cheers,
Pete.

Michael Droettboom-3 wrote:
  

This is a known issue with the contouring code. It's borrowed from an earlier plotting package called GIST, and assumes that the renderer can not handle compound polygons (for example, donut-shaped, with both an inner and outer edge). So instead, it draws "cuts" that go from the inner to the outer edge. When anti-aliasing is turned on, there is a slight overlap or "drawing twice" along this edge which creates what looks like a line. Unfortunately, this can't be remedied by plotting the contour fills and then plotting the contour edges over top (as is done is contourf_demo.py).

As of matplotlib 0.98.x, matplotlib itself can handle compound paths, so we no longer need the cuts. I've made a few attempts at updating the contouring code to avoid them, but got nowhere. The contouring code is very opaque, almost magical code, and most who've dared to go in have barely made it out alive... :wink: That said, a fresh pair of eyes may have what it takes...

As for a workaround, you could render your contour as opaque, save that out as an image and read it back in. The code to do that won't be pretty, but it just might work.

Cheers,
Mike

kippertoffee wrote:
    

Hello,

I am attempting to overlay a filled contour over a custom image.

I have managed to get something basic working, but i have encountered a
problem:

When the contourf plot is set to semi-transparent there are visible lines
joining the bottom of the plot and the filled contour edges. I have
attached
an image of the plot.

ImageShack - Best place for all of your image hosting and image sharing needs spam.png

The code i have used is below; please bear in mind I am not a programmer,
so
if the code seems botched, that's because it is.

###############################

import matplotlib.pyplot as plt
from pylab import *
try:
    import Image
except ImportError, exc:
    raise SystemExit("PIL must be installed to run this example")

lena = Image.open('lena.jpg')
dpi = rcParams['figure.dpi']
figsize = lena.size[0]/dpi, lena.size[1]/dpi

fig = plt.figure(figsize=figsize)
#fig.patch.set_alpha(0.5)
ax = fig.add_subplot(111)
#ax.patch.set_alpha(0.5)

ax.imshow(lena, origin='lower')

ax.contourf(z2,[10,15,20,25,30,35,40,45,50,55,60,65],alpha=0.7)
show()
############################

Can anyone help me with this problem?

Thanks for reading,
Pete.
      
--
Michael Droettboom
Science Software Branch
Operations and Engineering Division
Space Telescope Science Institute
Operated by AURA for NASA

5

No michael, that is what I was suggesting. Shame it doesn’t work.

I’m looking at using Enthoughts Chaco to do it. It’s takes a bit more to get it doing what I want though, and I don’t know if it does alpha transparency at all.

Thanks for your help.

Pete

2008/8/6 Michael Droettboom <mdroe@…86…>

···

It doesn’t seem to matter whether the filled contours are drawn with edges or not – the cuts are still visible because they’re caused by over-drawing of the fill. (You can set linewidth=0 to try this.) But maybe I misunderstand your question.

Cheers,

Mike

kippertoffee wrote:

Thank you for your reply Michael.

Is it possible to make the lines in the contourf plot completely transparent

using an alpha setting? That would make a reasonable workaround as I intend

to overlay black contours anyway.

I’ve had a look at the source but it is way beyond me; I am a mere dabbler.

Cheers,

Pete.

Michael Droettboom-3 wrote:

This is a known issue with the contouring code. It’s borrowed from an earlier plotting package called GIST, and assumes that the renderer can not handle compound polygons (for example, donut-shaped, with both an inner and outer edge). So instead, it draws “cuts” that go from the inner to the outer edge. When anti-aliasing is turned on, there is a slight overlap or “drawing twice” along this edge which creates what looks like a line. Unfortunately, this can’t be remedied by plotting the contour fills and then plotting the contour edges over top (as is done is contourf_demo.py).

As of matplotlib 0.98.x, matplotlib itself can handle compound paths, so we no longer need the cuts. I’ve made a few attempts at updating the contouring code to avoid them, but got nowhere. The contouring code is very opaque, almost magical code, and most who’ve dared to go in have barely made it out alive… :wink: That said, a fresh pair of eyes may have what it takes…

As for a workaround, you could render your contour as opaque, save that out as an image and read it back in. The code to do that won’t be pretty, but it just might work.

Cheers,

Mike

kippertoffee wrote:

Hello,

I am attempting to overlay a filled contour over a custom image.

I have managed to get something basic working, but i have encountered a

problem:

When the contourf plot is set to semi-transparent there are visible lines

joining the bottom of the plot and the filled contour edges. I have

attached

an image of the plot.

http://img232.imageshack.us/my.php?image=spambs6.png spam.png

The code i have used is below; please bear in mind I am not a programmer,

so

if the code seems botched, that’s because it is.

###############################

import matplotlib.pyplot as plt

from pylab import *

try:

import Image

except ImportError, exc:

raise SystemExit("PIL must be installed to run this example")

lena = Image.open(‘lena.jpg’)

dpi = rcParams[‘figure.dpi’]

figsize = lena.size[0]/dpi, lena.size[1]/dpi

fig = plt.figure(figsize=figsize)

#fig.patch.set_alpha(0.5)

ax = fig.add_subplot(111)

#ax.patch.set_alpha(0.5)

ax.imshow(lena, origin=‘lower’)

ax.contourf(z2,[10,15,20,25,30,35,40,45,50,55,60,65],alpha=0.7)

show()

############################

Can anyone help me with this problem?

Thanks for reading,

Pete.

–

Michael Droettboom

Science Software Branch

Operations and Engineering Division

Space Telescope Science Institute

Operated by AURA for NASA

6

I believe chaco uses the same contouring routine as mpl does, though I
haven't verified this recently.

JDH

···

On Wed, Aug 6, 2008 at 1:28 PM, peter websdell <flyingdeckchair@...982...> wrote:

No michael, that is what I was suggesting. Shame it doesn't work.

I'm looking at using Enthoughts Chaco to do it. It's takes a bit more to get
it doing what I want though, and I don't know if it does alpha transparency
at all.