How to work with single bands of a multiband image?

[gilbertocamara]

Hi @edzer

I have a multiband image and I am trying to process each band independently. However, I am at a loss to find out how to do so.
Please see the following MWE

image_path <- "https://www.dropbox.com/s/az9oxl1tcw1mz84/SENTINEL-2_MSI_20LLQ_2020-06-04_2021-08-26_variance_v1.tif?dl=1"

# read image using stars
image_st <- stars::read_stars( image_path, proxy = FALSE)
# set the labels
labels <- c("Water", "ClearCut_Burn", "ClearCut_Soil", "ClearCut_Veg",
            "Forest", "Wetlands")
names(labels) <- seq_along(labels)

# rename stars object dimensions to labels
image_st <- stars::st_set_dimensions(image_st, "band", values = labels)

The following command, which processes all pixels of all bands, works fine.

scale <- 0.01
image_st <- image_st * scale

I can acess the an image subset, but not process it. The following commands work.

image_st[,,,"Water"] 
image_st[,,,"Water"] > 5.0

My problem is that I want to process each band independently and the following command does not work.

scale <- 0.01
image_st[,,,"Water"] <- image_st[,,,"Water"]  * scale

What is the correct way to process individual bands in a stars image?

Many thanks,
Gilberto

[kadyb]

How about this way?

idx = which(labels == "Water")
image_st[[1]][,,idx] = image_st[[1]][,,idx] * scale

[edzer]

Yes, that was also my first thought: use the fact that image_st[[1]] is an R array. The alternative (using labels) could split the array into a set of 6 2-dimensional arrays:

image_spl = split(image_st)
image_spl[["Water"]] = image_spl[["Water"]] * scale                                   

and if needed merge again by

merge(image_spl)

This works at the same level: [[ extracts the array, [[<- replaces it.

[edzer]

But I'd agree it would be nice if this would work with single brackets!

[gilbertocamara]

Thanks, @edzer, works!

[edzer]

But I'd agree it would be nice if this would work with single brackets!

This allows for

image_spl["Water"] = image_spl["Water"] * scale                                   

and does some checking of the dimensions.

[gilbertocamara]

Hi @edzer, the quandary between [ ] and [[ ]] is an inherent problem in R. Maybe the only suggestion I would give is to provide some examples in the documentation.

[edzer]

Great idea!