I have encountered a, to me, strange behaviour in the histogram function of QGIS (2.2.0 Valmiera on Windows 7). Basically the same image produces totally different histograms.
I might have missed something fairly obvious, but I'm not able to find out what, and I have not been able to find any previous questions on the matter.
I aim to do a "Black object subtraction" in order to correct for atmospheric noise in satellite images. Without getting too technical about that, I'm looking for the lowest pixel values with a frequency of at least 1000 pixels (a suggested method according to the literature).
When I calculate a histogram for my SPOT5 image, the maximum frequency of any pixel value is below 1000 (accept for 0, that indicates no data). I find that suspiciously low, as the image is 7900*7900 pixels.
However, if I save the picture to with QGIS, just as it is, without any modification (see attached screen dump of save dialogue), I get a totally different histogram, that looks way more plausible to me (see image above).
How come that the basically the same raster produce totally different histograms? Is this a bug in QGIS? Faulty handling of images by me? Two different ways of rendering histograms?