qgis – Extracting Pixels from Raster Based on Tolerance Value Similar to Photoshop

extractqgisraster

I'm new with raster in QGIS. I have a three-band (RGB) georeferenced raster similar to this: (Reference)

And I would like to extract the pixels with similar color like in the following image by given a tolerance value as in Photoshop. In Photoshop, you can change color range to be selected using tolerance value. Two different results depending on the value 32 and 70 are seen in the two images below. (In both images, the darker blue area was clicked by "The Magic Wand Tool")

I was only able to find the following two posts that are most relevant to this topic. But they don't meet my need.

So, how can I extract the pixels using a color tolerance, as in Photoshop? It doesn't have to be a tolerance value, my ultimate goal is to extract pixels of similar color, but, somehow, I need to be able to set the similarity.

Best Answer

There is the Magic Wand plugin in QGIS that works similar to the magic wand tool known from graphic software. Install the plugin, then simply click on the map canvas and the plugin creates a polygon layer with the selection.

You could use this polygon to rasterize and use the Raster Calculator to mask the colors in the initial raster - or simply use the polygon layer for that.

You can change the Accuracy (Fast to Precise) and Color Threshold (Ambiguous to Strict).

One single click on the Ottoman Empire was enough to create Polygon styled in red hachures: $C:\Users\DU\Desktop\temp_new\320.tif$

Related Solutions

[GIS] Extracting data, not colors, from geotiffs in R

They are the values in the file. How they correspond to measurements is unknown, and would have to be specified in external metadata.

A GeoTIFF can easily store decimal numbers in bands, and you can get the range simply enough. Here I read in a 3 band raster using stack and check the first band:

> dd = stack("d.tif")
> range(values(dd)[,1])
[1]   0.0000 801.1061
>

If you have information that says how you map your band-1 values to measurements then you just have to operate on the raster values. For example, to scale values from 0 to max(d1) to 0 to 2*pi:

d1 = dd[[1]] # get first band
d1 = d1 * 2 * pi / max(values(d1))

If your TIFF is one byte per value, and you want to scale it to 1 to 20, for example:

d1 = 1 + d1 * (19/255)

should do it.

[GIS] Extracting Spatial Points that match Raster Value using R

You need to create a logic test to select those points. With an example:

library(raster)
library(sp)

set.seed(123)

# raster creation
r <- raster()

# values 0 and 1
r <- setValues(r, sample(x = 0:1, size = ncell(r), replace = T))

# point creation
pts <- spsample(as(r@extent,"SpatialPolygons"),100,type='random')

r is a binary raster and pts is a point layer with 100 features, let's see raster values of points position:

head(extract(r,pts))
## [1] 0 1 0 0 1 0

We have 0 and 1 values, to select only the points with position over values equal to 0, it's necessary to create a logic vector and select rows with this vector:

ones <- which(extract(r == 1,pts) == 1)

pts2 <- pts[ones,]

Now, let's see values of this new layer:

head(extract(r,pts2))

## [1] 1 1 1 1 1 1

Best Answer

Related Solutions

[GIS] Extracting data, not colors, from geotiffs in R

[GIS] Extracting Spatial Points that match Raster Value using R

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