CODEWITHSUNDEEP
rpngraster
jhhwilliams
jhhwilliams

Reputation: 2582

How to remove padding added to a PNG image created with writeGDAL

I am using writeGDAL to export raster data in PNG format to use as an image overlay on Google Maps. The image therefore needs to have the correct aspect ratio and must fit the raster extent exactly.

When I export the UTM-projected raster the result is as expected but after I project to the LatLong system the generated PNG has padding right round the raster area.

What do I need to do to get rid of this padding?

UTM image without padding LatLong image with padding (black)

Below is sample code which creates 2 images that demonstrate the problem.

library(raster)
library(rgdal)

r <- raster(xmn=742273.5, xmx=742702.5, ymn=6812515.5, ymx=6812995.5, ncols=144, nrows=161)
r <- setValues(r, 1:ncell(r))
projection(r) <- CRS('+proj=utm +zone=35 +south +datum=WGS84 +units=m +no_defs')
pr <- projectRaster(r, crs='+proj=longlat +datum=WGS84 +no_defs')

#Coerce to SpatialPixelsDataFrame and prepare for writeGDAL
rSpdf <- as(r, 'SpatialPixelsDataFrame')
prSpdf <- as(pr, 'SpatialPixelsDataFrame')
rSpdf$colors <- as.numeric(cut(rSpdf$layer, breaks = 255))
prSpdf$colors <- as.numeric(cut(prSpdf$layer, breaks = 255))
colorTable <- list(colorRampPalette(c('red', 'yellow', 'green4'))(256))

#Export in PNG format using writeGDAL
writeGDAL(rSpdf[, 'colors'], 'utm.png', drivername = 'PNG', type = 'Byte', mvFlag = 0, colorTables = colorTable)
writeGDAL(prSpdf[, 'colors'], 'geo.png', drivername = 'PNG', type = 'Byte', mvFlag = 0, colorTables = colorTable)

#Optionally, the rasters can be exported to view in a spatial package (eg SAGA-GIS)
#writeRaster(r, filename='utm.tif', format="GTiff", overwrite=TRUE)
#writeRaster(pr, filename='geo.tif', format="GTiff", overwrite=TRUE)

Upvotes: 0

Views: 450

Answers (1)

jhhwilliams
jhhwilliams

Reputation: 2582

By converting the projected raster to points and then coercing the points to a SpatialPixelsDataFrame (instead of coercing the raster) the padding is removed.

PNG without padding

library(raster)
library(rgdal)

r <- raster(xmn=742273.5, xmx=742702.5, ymn=6812515.5, ymx=6812995.5, ncols=144, nrows=161)
r <- setValues(r, 1:ncell(r))
projection(r) <- CRS('+proj=utm +zone=35 +south +datum=WGS84 +units=m +no_defs')
pr <- projectRaster(r, crs='+proj=longlat +datum=WGS84 +no_defs')

points <- rasterToPoints(pr, spatial = TRUE)
prSpdf <- as(points, 'SpatialPixelsDataFrame')

prSpdf$colors <- as.numeric(cut(prSpdf$layer, breaks = 10))
colorTable <- list(colorRampPalette(c('red', 'yellow', 'green4'))(11))

writeGDAL(prSpdf[, 'colors'], 'geo.png', drivername = 'PNG', type = 'Byte', mvFlag = 0, colorTables = colorTable)

Upvotes: 1

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