[R] graphics useRaster check_irregular definition for date or posix

[Paul Murrell]

Hi

Feeding R-help back in, in case my suggestions might be of use to 
someone else ...

Thanks for the examples - so the main benefit you are looking for is the 
labelling on the axes (date labels) ?

If you are just trying to avoid the annoying white lines, it may just be 
your PNG/PDF viewer;  see ...

http://cran.stat.auckland.ac.nz/doc/FAQ/R-FAQ.html#Why-are-there-unwanted-borders

Even so, making this work for date values for x/y seems like a useful 
thing.  However, ...

Neither your "fix" nor my "fix" actually works for the example that you 
have provided.  It just reveals another problem within image.default(), 
which is the calculation of "midpoints" ...

     if (length(x) > 1 && length(x) == nrow(z)) { # midpoints
         dx <- 0.5*diff(x)
         x <- c(x[1L] - dx[1L], x[-length(x)] + dx,
                x[length(x)] + dx[length(x)-1])
     }

This calculation does NOT produce the desired result for "Date"s (the 
diff() of the resulting modified 'x' is no longer regular).

So this needs a bit more thought - let me know if you come up with a fix 
for that calculation before me :)

Paul

On 15/04/21 4:05 am, cdanek using posteo.de wrote:
> Hi Paul
> 
> Thanks for your answer. I just answer to you, I dont know if this is 
> correct (also the maillist?)
> 
> x <- y <- seq(as.Date("2020-1-1"), as.Date("2020-12-31"), l=12)
> 
> z <- array(rnorm(length(x)*length(y)), c(length(x), length(y)))
> image(x, y, z, useRaster=FALSE) # ok
> 
> image(x, y, z, useRaster=TRUE) # error: ‘useRaster = TRUE’ can only be used with a regular grid
> 
> x <- y <- seq_len(12)
> 
> image(x, y, z, useRaster=FALSE) # ok
> 
> image(x, y, z, useRaster=TRUE) # ok
> 
> The difference is the plot quality: useRaster = TRUE yields better quality as it omits strange white lines (in both png and pdf). I did not find a pattern when those white lines appear and when not. Hence, I cannot provide a reproducable example in which you can see the differences induced by the useRaster argument. In simple examples like the one above there is no difference between useRaster true and false plots.
> 
> Do you have a suggestion on how to continue?
> 
> Thanks a lot,
> Chris
> 
> Am 14.04.2021 04:37 schrieb Paul Murrell:
> 
>> Hi
>>
>> I doubt it is intended (to deliberately exclude "difftime" objects).
>>
>> Can you please supply a full image() example (with 'x' and/or 'y' as Dates and a 'z') ?  So that I can see what ...
>>
>> image(x, y, z, useRaster=FALSE)
>>
>> ... looks like, so I can see what you want ...
>>
>> image(x, y, z, useRaster=TRUE)
>>
>> ... to look like.
>>
>> I also wonder whether switching to ...
>>
>> dx[1][rep(1, length(dx))]
>>
>> ... might be better than switching to ...
>>
>> as.numeric(dx)
>>
>> It produces the same result for "difftime" objects, and may have a better chance of working better with other objects (although I confess that not having thought of using a "difftime" for 'x' I am also failing to think of further possibilities for 'x').
>>
>> Paul
>>
>> On 14/04/21 3:11 am,cdanek using posteo.de  <mailto:cdanek using posteo.de>  wrote:
>>> Hi The function `check_irregular()` defined within 
>>> `graphics::image.default()` checks if the `useRaster` argument for 
>>> `graphics::image()` can be true or must be false. According to this 
>>> function, the following example vector is irregular: ``` time <- 
>>> seq(as.Date("2020-1-1"), as.Date("2020-12-31"), l=12) 
>>> check_irregular(time, time) # TRUE ``` In my view, this is not 
>>> correct. In this case, the `all.equal`-call does not evaluate to true 
>>> due to the special class of `dx` (or `dy`). If I slightly rewrite the 
>>> function as ``` my_check_irregular <- function (x, y) { dx <- 
>>> as.numeric(diff(x)) dy <- as.numeric(diff(y)) (length(dx) && 
>>> !isTRUE(all.equal(dx, rep(dx[1], length(dx))))) || (length(dy) && 
>>> !isTRUE(all.equal(dy, rep(dy[1], length(dy))))) } ``` the correct 
>>> answer is obtained (i.e. that the input vector is not irregular based 
>>> on the rational behind `all.equal`): ``` my_check_irregular(time, 
>>> time) # FALSE ``` The same applies to POSIX* objects. I was wondering 
>>> if this is intended or not? Thanks a lot for any answer, Chris 
>>> [[alternative HTML version deleted]] 
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-- 
Dr Paul Murrell
Department of Statistics
The University of Auckland
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