Use QGIS to identify and count pixel colour in a photo

Question

I am trying to use QGIS to analyse the colour component of a photo. I have imported the photo as a raster layer, created a polygon of the area in question and have tried to use zonal statistics but this doesn't appear to work. When I use the zonal statistics function, I fill in all the relevant boxes and click ok. Then the message window closes. That's it. When I have tried using the function on maps I already have, after clicking ok the progress bar came up and after it finished I could find the resulting stats. When I tried it on my photo, absolutely nothing happens. The window just closes, there are no stats anywhere.

Ideally, the stats I would like to produce are the number of pixels in the polygon, and the number of pixels of each colour. Or, to assign the colours into similar groups say 0-25, 25-50, 50-75 etc up to 255. Of course, this is made more challenging by the fact that a photo used as a raster layer is made up of 3 colour bands and, as yet, I haven't managed to separate the bands.

Answer 1

You want to count the number of discrete pixel colours inside an area? As far as I know, that can't be done with zonal statistics.

e.g.

Polygon 1 - area is 1000 pixels
500 pixels are (122,135,21)
400 pixels are (22,132,178)
100 pixels are (2,156,99)

Doing that on an raw (unclassified) rgb image will give a HUGE number of possible colours. A polygon with an area of 1000 pixels could easily have 900 unique colours, for example. Most pixel colours will appear only once, and a few will appear twice.

A python/gdal script could be written to count unique colours, but it would be slow and memory-intensive for that very reason.

You can get around this by using a paletted image to "merge" similar colours into a single value. This means that greens would all appear as one value, greys another value, and so on.

(If you're familiar with Photoshop/GIMP, this is the same as reducing an image to a fixed palette size e.g. 16 colours, where each pixel is 'rounded' to the nearest colour from a representative palette of 16 colours)

Try "convert rgb to paletted" (raster > conversion > rgb to pct). More info here

This will classify pixels into groups of similar colour.

Now the problem is easier to address.

I took an aerial image and reduced it to 4 colours.

Then I ran gdalinfo to get a histogram of the values. In Linux, use the terminal, in Windows, use the OSGeo4W shell.

gdalinfo -hist /path/to/my.tiff

This gave me

313730340 154657 11795972 160083079 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

So there are 313,730,340 pixels with value 0. And only 154,657 with value 1.

You'll need to work out which value equates to which colour, though.

There's also a K-Means raster classifier in Orfeo Toolbox, this does a simplification of colours but uses a different algorithm.

Answer 2

You are searching at the wrong place. Zonal statistics are in the attributes table of polygon vector layer. Check it out at the next image:

where attributes table has all possible relevant stats for zonal statistics into ocher polygon. However, it's only for first band (red). For all bands, you should select them separately in band selector of 'Zonal Statistics' window.

Editing Note:

Based in your comment, you can be interested in r.report (GRASS geoalgorithm) of Processing Tool Box. However, I think that you need to use each band separately (one GRASS limitation). In my next example,

I selected:

1. Raster layer to report
2. 'c' option for count cells
3. 'Select extent on Canvas', by dragging with the mouse, at the polygon area

After click in 'Run', I got:

the number of pixels (band 1) for each value. However, you can change the number of fp subranges to collect stats for grouping results. I hope that it helps.