Plot to a device and examine the plot?

Hi All,

I want to inspect the content of a plot generated by another function.

For example:

plot.new()
polygon(c(.5, .5, .75, .8), c(.25, .3, .4, .5))

A polygon will be drawn. If I do not know what has been done to generate
the plot, is it possible to query the content in the active device?

Regards,
Shu Fai

[[alternative HTML version deleted]]

This question is not clear to me. What is it you hope to retrieve from the
device?

Note that the type of device in your example is system-dependent. The
content in a png() would be different than the content in a win.graph()
device.

On October 15, 2023 8:04:00 AM PDT, Shu Fai Cheung <
shufai.cheung at gmail.com> wrote:

Hi All,

I want to inspect the content of a plot generated by another function.

For example:

plot.new()
polygon(c(.5, .5, .75, .8), c(.25, .3, .4, .5))

A polygon will be drawn. If I do not know what has been done to generate
the plot, is it possible to query the content in the active device?

Regards,
Shu Fai

      [[alternative HTML version deleted]]

Sorry that I did not make my question clear enough.

If the device is file based (e.g., a PNG file), then I believe I can do
that, by using functions that can inspect an image. This is the solution I
originally wanted to try. However, it requires creating a file in the
process.

I would like to see whether it is possible to inspect the canvas if the
device is an on-screen one, like the pop-up window in R for Windows.

Regards,
Shu Fai


On Sun, Oct 15, 2023 at 11:31?PM Jeff Newmiller <jdnewmil at dcn.davis.ca.us>
wrote:

This question is not clear to me. What is it you hope to retrieve from
the device?

Note that the type of device in your example is system-dependent. The
content in a png() would be different than the content in a win.graph()
device.

On October 15, 2023 8:04:00 AM PDT, Shu Fai Cheung <
shufai.cheung at gmail.com> wrote:

Hi All,

I want to inspect the content of a plot generated by another function.

For example:

plot.new()
polygon(c(.5, .5, .75, .8), c(.25, .3, .4, .5))

A polygon will be drawn. If I do not know what has been done to generate
the plot, is it possible to query the content in the active device?

Regards,
Shu Fai

      [[alternative HTML version deleted]]

Let me clarify my question:

plot.new()
polygon(c(.5, .5, .75, .8), c(.25, .3, .4, .5))

If the device is an on-screen device, can I check whether a particular area
has anything drawn on it, or, to be precise, whether the color of a
particular area has all pixels equal to the background color. That is, if
the background is white, can I know whether a particular area is white?

E.g.,  in the case above, the area bounded by rect(0, 0, .25, .25) is
completely white, while the area bounded by rect(0, 0, .75, .75) is not
because part of border of the polygon, black by default, has been drawn in
this area.

If the device is an image file, then I can check the color of a pixel. I
would like to know whether I can do the same with an on-screen device.

On 15/10/2023 12:05 p.m., Shu Fai Cheung wrote:

 > Let me clarify my question:
 >
 > plot.new()
 > polygon(c(.5, .5, .75, .8), c(.25, .3, .4, .5))
 >
 > If the device is an on-screen device, can I check whether a 

particular area

 > has anything drawn on it, or, to be precise, whether the color of a
 > particular area has all pixels equal to the background color. That is, if
 > the background is white, can I know whether a particular area is white?
 >
 > E.g., in the case above, the area bounded by rect(0, 0, .25, .25) is
 > completely white, while the area bounded by rect(0, 0, .75, .75) is not
 > because part of border of the polygon, black by default, has been 

drawn in

 > this area.
 >
 > If the device is an image file, then I can check the color of a pixel. I
 > would like to know whether I can do the same with an on-screen device.
 >

I think the answer is that you can't do that in general. However, in
general you can copy a plot to a different device using dev.copy() and
examine it there. It won't be pixel-by-pixel identical, but will
contain the same components, likely with slightly different scaling and
positioning if the new device isn't the same as the old one.

You can also save the commands that drew the plot using recordPlot() and
redraw it using replayPlot() (which is essentially what dev.copy()
does), but the format of the object saved by recordPlot() is not
documented, and is subject to change with R version changes.

Duncan Murdoch

On 15/10/2023 12:05 p.m., Shu Fai Cheung wrote:

Let me clarify my question:

plot.new()
polygon(c(.5, .5, .75, .8), c(.25, .3, .4, .5))

If the device is an on-screen device, can I check whether a particular area
has anything drawn on it, or, to be precise, whether the color of a
particular area has all pixels equal to the background color. That is, if
the background is white, can I know whether a particular area is white?

E.g.,  in the case above, the area bounded by rect(0, 0, .25, .25) is
completely white, while the area bounded by rect(0, 0, .75, .75) is not
because part of border of the polygon, black by default, has been drawn in
this area.

If the device is an image file, then I can check the color of a pixel. I
would like to know whether I can do the same with an on-screen device.

I think the answer is that you can't do that in general.  However, in
general you can copy a plot to a different device using dev.copy() and
examine it there.  It won't be pixel-by-pixel identical, but will
contain the same components, likely with slightly different scaling and
positioning if the new device isn't the same as the old one.

You can also save the commands that drew the plot using recordPlot() and
redraw it using replayPlot() (which is essentially what dev.copy()
does), but the format of the object saved by recordPlot() is not
documented, and is subject to change with R version changes.

Duncan Murdoch

Hi

You could also look at dev.capture(), depending on which screen device
you are using.

Paul

On 16/10/23 05:24, Duncan Murdoch wrote:

On 15/10/2023 12:05 p.m., Shu Fai Cheung wrote:

 > Let me clarify my question:
 >
 > plot.new()
 > polygon(c(.5, .5, .75, .8), c(.25, .3, .4, .5))
 >
 > If the device is an on-screen device, can I check whether a

particular area

 > has anything drawn on it, or, to be precise, whether the color of a
 > particular area has all pixels equal to the background color. That is, if
 > the background is white, can I know whether a particular area is white?
 >
 > E.g., in the case above, the area bounded by rect(0, 0, .25, .25) is
 > completely white, while the area bounded by rect(0, 0, .75, .75) is not
 > because part of border of the polygon, black by default, has been

drawn in

 > this area.
 >
 > If the device is an image file, then I can check the color of a pixel. I
 > would like to know whether I can do the same with an on-screen device.
 >

I think the answer is that you can't do that in general. However, in
general you can copy a plot to a different device using dev.copy() and
examine it there. It won't be pixel-by-pixel identical, but will
contain the same components, likely with slightly different scaling and
positioning if the new device isn't the same as the old one.

You can also save the commands that drew the plot using recordPlot() and
redraw it using replayPlot() (which is essentially what dev.copy()
does), but the format of the object saved by recordPlot() is not
documented, and is subject to change with R version changes.

Duncan Murdoch

--
Dr Paul Murrell
Te Kura Tatauranga | Department of Statistics
Waipapa Taumata Rau | The University of Auckland
Private Bag 92019, Auckland 1142, New Zealand
64 9 3737599 x85392
paul at stat.auckland.ac.nz
www.stat.auckland.ac.nz/~paul/