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Tile resolution #5

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oloverm opened this issue Mar 29, 2021 · 12 comments
Closed

Tile resolution #5

oloverm opened this issue Mar 29, 2021 · 12 comments

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@oloverm
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oloverm commented Mar 29, 2021

Hi,

The tiles downloaded using get_tiles() are a bit blurry. Does OpenStreetMap just not allow downloading of higher-resolution tiles? Or could they be sharper?

For example, this is the OpenStreetMap website on the left, and tiles downloaded on the right (same zoom level):

image
@rCarto
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rCarto commented Mar 30, 2021

Hello,

  1. I think this blur is due to the re-projection of the map. Tiles are displayed in OSM and downloaded via maptiles in EPSG:3857 projection.
    If x (in get_tiles()) uses another coordinates reference system then a projection is applied to match the CRS of x and the tiles are kind of resampled in the process.
    The solution to obtain a non-blurred image is to use EPSG:3857 as input for get_tiles().

  2. The export process can also alter the tiles quality. If you want to obtain exactly the same image you'll have to export the map using the tiles dimensions:

library(maptiles)
library(sf)
#> Linking to GEOS 3.7.1, GDAL 3.1.2, PROJ 7.1.0
zone_4326 <- st_as_sfc(st_bbox(c(xmin = 14.63128, xmax = 15.20052, 
                                 ymin = 54.93529, ymax = 55.31899), 
                               crs = 4326))
zone_3857 <- st_transform(zone_4326, 3857)
tiles_3857 <- get_tiles(x = zone_3857, zoom = 11)
dim(tiles_3857)
#> [1] 1280 1024    3
png("export.png", width = 1024, height = 1280)
plot_tiles(tiles_3857)
dev.off()
#> png 
#>   2

Created on 2021-03-30 by the reprex package (v1.0.0)
export

@oloverm
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oloverm commented Mar 31, 2021

Hmm yes yours is perfectly crisp. I wonder if it might be a computer/platform issue. Are you on a Mac, could that mean anything?

I ran basically the same code as you, but I still get a fuzzy image. I had to add par(mar = c(0,0,0,0)) because otherwise I got a big margin that I thought might be causing the pixel dimensions not to match up. But even without that margin, it's still not right.

library(maptiles)
library(sf)
zone_4326 <- st_as_sfc(st_bbox(c(xmin = 14.63128, xmax = 15.20052, 
                                 ymin = 54.93529, ymax = 55.31899), 
                               crs = 4326))
zone_3857 <- st_transform(zone_4326, 3857)
tiles_3857 <- get_tiles(x = zone_3857, zoom = 11)

tile_dim <- dim(tiles_3857)

png("export.png", width = tile_dim[2], height = tile_dim[1])
par(mar = c(0,0,0,0))  # to remove the thick white margin
plot_tiles(tiles_3857)
dev.off()

export

@rCarto
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rCarto commented Mar 31, 2021

I may be wrong but I don't think it's a platform problem.
Would you mind trying the dev version of the package (remotes::install_github("riatelab/maptiles")). I made small changes to plot_tiles(), setting margins is not needed anymore. My previous example used this version, it may explain the differences. I've also added my session info.

library(maptiles)
library(sf)
#> Linking to GEOS 3.7.1, GDAL 3.1.2, PROJ 7.1.0
zone_4326 <- st_as_sfc(st_bbox(c(xmin = 14.63128, xmax = 15.20052, 
                                 ymin = 54.93529, ymax = 55.31899), 
                               crs = 4326))
zone_3857 <- st_transform(zone_4326, 3857)
tiles_3857 <- get_tiles(x = zone_3857, zoom = 11)
tile_dim <- dim(tiles_3857)
png("export.png", width = tile_dim[2], height = tile_dim[1])
plot_tiles(tiles_3857)
dev.off()
#> png 
#>   2

Created on 2021-03-31 by the reprex package (v1.0.0)

Session info 
sessioninfo::session_info()
#> ─ Session info ───────────────────────────────────────────────────────────────
#>  setting  value                       
#>  version  R version 4.0.4 (2021-02-15)
#>  os       Debian GNU/Linux 10 (buster)
#>  system   x86_64, linux-gnu           
#>  ui       X11                         
#>  language (EN)                        
#>  collate  fr_FR.UTF-8                 
#>  ctype    fr_FR.UTF-8                 
#>  tz       Europe/Paris                
#>  date     2021-03-31                  
#> 
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#> 
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#> [2] /usr/local/lib/R/site-library
#> [3] /usr/lib/R/site-library
#> [4] /usr/lib/R/library

export

@oloverm
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oloverm commented Mar 31, 2021

No luck on the dev version either, unfortunately:

library(maptiles)
library(sf)
#> Linking to GEOS 3.9.0, GDAL 3.2.1, PROJ 7.2.1
zone_4326 <- st_as_sfc(st_bbox(c(xmin = 14.63128, xmax = 15.20052, 
                                 ymin = 54.93529, ymax = 55.31899), 
                               crs = 4326))
zone_3857 <- st_transform(zone_4326, 3857)
tiles_3857 <- get_tiles(x = zone_3857, zoom = 11)
tile_dim <- dim(tiles_3857)
png("export.png", width = tile_dim[2], height = tile_dim[1])
plot_tiles(tiles_3857)
dev.off()
#> png 
#>   2

Created on 2021-03-31 by the reprex package (v1.0.0)

Session info 
sessioninfo::session_info()
#> - Session info ---------------------------------------------------------------
#>  setting  value                       
#>  version  R version 4.0.4 (2021-02-15)
#>  os       Windows 10 x64              
#>  system   x86_64, mingw32             
#>  ui       RTerm                       
#>  language (EN)                        
#>  collate  English_United Kingdom.1252 
#>  ctype    English_United Kingdom.1252 
#>  tz       Europe/Paris                
#>  date     2021-03-31                  
#> 
#> - Packages -------------------------------------------------------------------
#>  package     * version date       lib source                            
#>  assertthat    0.2.1   2019-03-21 [1] CRAN (R 4.0.3)                    
#>  backports     1.2.1   2020-12-09 [1] CRAN (R 4.0.3)                    
#>  class         7.3-18  2021-01-24 [2] CRAN (R 4.0.4)                    
#>  classInt      0.4-3   2020-04-07 [1] CRAN (R 4.0.3)                    
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#>  codetools     0.2-18  2020-11-04 [2] CRAN (R 4.0.4)                    
#>  crayon        1.4.1   2021-02-08 [1] CRAN (R 4.0.4)                    
#>  curl          4.3     2019-12-02 [1] CRAN (R 4.0.3)                    
#>  DBI           1.1.1   2021-01-15 [1] CRAN (R 4.0.4)                    
#>  digest        0.6.27  2020-10-24 [1] CRAN (R 4.0.4)                    
#>  dplyr         1.0.5   2021-03-05 [1] CRAN (R 4.0.4)                    
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#>  ellipsis      0.3.1   2020-05-15 [1] CRAN (R 4.0.3)                    
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export

@rCarto
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rCarto commented Mar 31, 2021 •
edited

All right, I checked on mac and windows machines, aaaaand you were right, it's definitely a platform problem.
You can try ragg::agg_png() as an alternative to png(), I had good results with it on windows.
My feeling is that the problem is not linked to maptiles itself but more generally to raster export. I'll try to export other raster objects (from raster and graphics::image())) to be sure.

See also : https://www.jumpingrivers.com/blog/r-graphics-cairo-png-pdf-saving/

@rhijmans
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rhijmans commented Apr 20, 2021 •
edited

You can also write a png file like this

writeRaster( tiles_3857, "tiles_3857.png", NAflag=NA, overwrite=TRUE)

@Nowosad
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Nowosad commented Jul 17, 2021

Hey @rCarto - I have tried a few graphic devices on Fedora - all return a slightly blurry image... Could you share your resulting terra object? If it is identical to mine, then we (probably) could confirm that it is not related to data downloading...

@rCarto
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rCarto commented Jul 20, 2021

Hi @Nowosad , this is what I get:

library(maptiles)
library(sf)
#> Linking to GEOS 3.7.1, GDAL 3.1.2, PROJ 7.1.0
library(terra)
#> terra version 1.3.4
zone_4326 <- st_as_sfc(st_bbox(c(xmin = 14.63128, xmax = 15.20052, 
                                 ymin = 54.93529, ymax = 55.31899), 
                               crs = 4326))
zone_3857 <- st_transform(zone_4326, 3857)
tiles_3857 <- get_tiles(x = zone_3857, zoom = 10, cache = "cache")

# png export via png() & plot_tiles()
tile_dim <- dim(tiles_3857)
png("export_1.png", width = tile_dim[2], height = tile_dim[1])
plot_tiles(tiles_3857, smooth = FALSE)
dev.off()
#> png 
#>   2

# png export via writeRaster()
terra::writeRaster(tiles_3857, "export_2.png", NAflag = NA, overwrite = TRUE)
#> Warning in x@ptr$writeRaster(opt): GDAL Message 6: driver MEM does not support
#> creation option PHOTOMETRIC

# tiff export
terra::writeRaster( tiles_3857, "export.tif", NAflag = NA, overwrite = TRUE)

# rds export
saveRDS(object = wrap(tiles_3857),file = 'export.rds')
# rds import
tiles_3857_import <- rast(readRDS('export.rds'))

Created on 2021-07-20 by the reprex package (v2.0.0)

I don't think the warning from the second png export is relevant here.

Zip file that contains exported files :
export.zip

@Nowosad
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Nowosad commented Jul 22, 2021

Just to confirm - I got the same images as yours.

Interactive viewing returns, however, worse quality image. Below you can find two examples:

  1. Default behavior of plotRGB()

2021-07-22_16-46

  1. plotRGB() with maxcell = Inf - better, but still far from perfect

2021-07-22_16-47

@rCarto
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rCarto commented Jul 27, 2021

@Nowosad ,
plotRGB() plots the image entire on the whole graphic device.
If the raster is smaller than the device (in pixels) the image is zoomed-in:
image
if the raster is larger than the device, the image is zoomed-out :
image
In both cases the quality of the resulting image is lower than the original. This happens to every raster images but it's more visible with map tiles because of labels, roads and small elements. I don't think we can do anything about that.

However,
In the pixel branch of maptiles I've added a pixel argument to plot_tiles().
When pixel=TRUE, if the raster is smaller than the device, the raster is centered with margins:
image
if the raster is larger than the device, the raster is cropped:
image

What do you think?

@Nowosad
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Nowosad commented Jul 30, 2021

Neat. It works well.
(Just a side note: now I think of an impact can this whole issue have on mapping packages (e.g., mapsf or tmap - @mtennekes) ; or in different words - how they should behave/what can they do with it)

@rCarto
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rCarto commented Sep 23, 2021

see adjust arg in plot_tiles()

@rCarto rCarto closed this as completed Sep 23, 2021
This was referenced Oct 10, 2023
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@rCarto @Nowosad @rhijmans @oloverm