[R] Plot to a device and examine the plot?

Shu Fai Cheung @hu|@|@cheung @end|ng |rom gm@||@com
Sun Oct 15 18:05:30 CEST 2023

Let me clarify my question:

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.


Shu Fai

On Sun, Oct 15, 2023 at 11:44 PM Shu Fai Cheung <shufai.cheung using gmail.com>

> 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 using 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 using 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]]
>> >
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