matplotlib-users

[Matplotlib-users] dpi

[Nikolaus R. <Nik...@ra...>]

Hello,

I'm confused by the dpi property of figures that can be set in
matplotlibrc or passed to pyplot.figure().

It seems to me that dpi is really a property of the backend, not the
figure, and the only place to specify it ought to be when saving into a
bitmap file. 

For example, when showing a figure on the screen, I really want
matplotlib to use the physical dpi of the monitor rather than any
default value. When saving the figure in some vector graphics format, I
don't see what the meaning of the dpi is at all.

Am I missing something?

Best,

   -Nikolaus

-- 
 »Time flies like an arrow, fruit flies like a Banana.«

  PGP fingerprint: 5B93 61F8 4EA2 E279 ABF6  02CF A9AD B7F8 AE4E 425C

Re: [Matplotlib-users] dpi

[Damon M. <dam...@gm...>]

On Thu, Oct 11, 2012 at 4:00 PM, Nikolaus Rath <Nik...@ra...> wrote:
> When saving the figure in some vector graphics format, I
> don't see what the meaning of the dpi is at all.

Sure, I use `dpi=` all the time for vector formats. Purely because
when you make calls to `imshow`, you get a rasterised image embedded
in a figure with vector text and tickmarks and labels, for example.

-- 
Damon McDougall
http://www.damon-is-a-geek.com
B2.39
Mathematics Institute
University of Warwick
Coventry
West Midlands
CV4 7AL
United Kingdom

Re: [Matplotlib-users] dpi

[Nikolaus R. <Nik...@ra...>]

Damon McDougall <dam...@pu...> writes:
> On Thu, Oct 11, 2012 at 4:00 PM, Nikolaus Rath <Nik...@pu...> wrote:
>> When saving the figure in some vector graphics format, I
>> don't see what the meaning of the dpi is at all.
>
> Sure, I use `dpi=` all the time for vector formats. Purely because
> when you make calls to `imshow`, you get a rasterised image embedded
> in a figure with vector text and tickmarks and labels, for example.

matplotlib actually rescales the raw imshow data when saving to a vector
format? Why is that? I think it should embed the bitmap with full
resolution in the vector file and rely on the consumer of the vector
file to scale it to whatever resolution is supported by the display
device.


Best,

   -Nikolaus

-- 
 »Time flies like an arrow, fruit flies like a Banana.«

  PGP fingerprint: 5B93 61F8 4EA2 E279 ABF6  02CF A9AD B7F8 AE4E 425C

Re: [Matplotlib-users] dpi

[Jae-Joon L. <lee...@gm...>]

On Fri, Oct 12, 2012 at 3:39 AM, Nikolaus Rath <Nik...@ra...> wrote:
> matplotlib actually rescales the raw imshow data when saving to a vector
> format? Why is that? I think it should embed the bitmap with full
> resolution in the vector file and rely on the consumer of the vector
> file to scale it to whatever resolution is supported by the display
> device.

imshow supports interpolation and that's why rasterization comes in.
If you turn off interpolation (w/ interpolation="none"), the original
image will be embedded. Of course dpi has no meaning in this case.

However, I agree with you that dpi should be a property of the backend
only, not the figure. But I am not sure if this can be fixed soon. It
will be difficult and will take lots of effort I think.

-JJ

Re: [Matplotlib-users] dpi

[Nikolaus R. <Nik...@ra...>]

Jae-Joon Lee <lee...@pu...> writes:
> On Fri, Oct 12, 2012 at 3:39 AM, Nikolaus Rath <Nik...@pu...> wrote:
>> matplotlib actually rescales the raw imshow data when saving to a vector
>> format? Why is that? I think it should embed the bitmap with full
>> resolution in the vector file and rely on the consumer of the vector
>> file to scale it to whatever resolution is supported by the display
>> device.
>
> imshow supports interpolation and that's why rasterization comes in.
> If you turn off interpolation (w/ interpolation="none"), the original
> image will be embedded. Of course dpi has no meaning in this case.
>
> However, I agree with you that dpi should be a property of the backend
> only, not the figure. But I am not sure if this can be fixed soon. It
> will be difficult and will take lots of effort I think.

Yeah, that's what I feared. But in the mean time, are there any best
practices to minimize undesired effects like the one above? For example,
are there any other functions that need special parameters to not raster
their output when writing to a vector format? And is there a way to get
a figure on the screen with the right size when I don't know what dpi
the monitor is running with?

Best,

   -Nikolaus

-- 
 »Time flies like an arrow, fruit flies like a Banana.«

  PGP fingerprint: 5B93 61F8 4EA2 E279 ABF6  02CF A9AD B7F8 AE4E 425C

Re: [Matplotlib-users] dpi

[Jae-Joon L. <lee...@gm...>]

> Yeah, that's what I feared. But in the mean time, are there any best
> practices to minimize undesired effects like the one above? For example,
> are there any other functions that need special parameters to not raster
> their output when writing to a vector format? And is there a way to get
> a figure on the screen with the right size when I don't know what dpi
> the monitor is running with?

As I said, if you use interpolation="none" with your inshow, the
original image will be sent to the backends.

Figuring out the dpi of the screen, I have no clue at this moment.
Maybe this is something a gui expert can answer.

Regards,

-JJ

Re: [Matplotlib-users] dpi

[Pierre H. <pie...@cr...>]

Hi,

Le 19/10/2012 06:48, Jae-Joon Lee a écrit :
> Figuring out the dpi of the screen, I have no clue at this moment.
> Maybe this is something a gui expert can answer.
I'm certainly not a gui expert, but as a PyQt user, I know screen
resolution is indeed Python-accessible with PyQt. (I guess other
toolkits provide their own methods)

I've made a quick script that prints the screen X and Y resolution
(requires PyQt). Reference links to PyQt API docs are included.


In my case, it's 96 dpi, and that what I use in my matplotlibrc file for
the "figure.dpi" property. But I use a higher value (say 150) for
"savefig.dpi" so that I get better resolution when saving PNG images.

I agree with Nikolaus that the dpi value for displaying figures would be
better get by the software, if possible. Maybe a property like
figure.dpi='auto' should activate such a behavior. But this would
require many code duplicates, one for each gui toolkit.


Best,
Pierre