Modify raster values within QGIS raster style dialog

Question

I'd like to use the QGIS Raster Style dialog to display modified raster values. Specifically, I'd like to see a gradient using the square root of the raster's value.

I understand it's possible to use the raster calculator to transform raster values, but - if possible - I'd like to skip that step and apply the mathematical formula during the styling, and avoid creating a modified copy of the original raster.

This type of operation is possible with vector files, using data defined values for colour/width/etc. Is it possible to do this with rasters?

Answer 1

I never try but it might possible by using Virtual raster file (VRT) create with GDAL on which you can perform calculation without modifying your datasource or create a huge new raster file on disk (just a vrt xml file that you can open in QGis and do you style on it's band values).

for exemple your vrt file should look like this (no tested):

<VRTDataset rasterXSize="1000" rasterYSize="1000">
    <VRTRasterBand dataType="Float32" band="1" subClass="VRTDerivedRasterBand">>
        <Description>Magnitude</Description>
        <PixelFunctionType>sqrt</PixelFunctionType>
        <SimpleSource>
            <SourceFilename relativeToVRT="1">yourraster.tiff</SourceFilename>
            <SourceBand>1</SourceBand>
            <SrcRect xOff="0" yOff="0" xSize="1000" ySize="1000"/>
            <DstRect xOff="0" yOff="0" xSize="1000" ySize="1000"/>
        </SimpleSource>
    </VRTRasterBand>
</VRTDataset>

To have more detail have a look to the Virtual format.

Answer 2

It's a good question. To my knowledge there's no way to do that for the moment, but I hope some day it will be possible.

As a temporary solution, you can try to convert your raster data to points and style them so they each look like boundaryless square of a size that equals the pixel spacing.

You can then use all vector styling tools you need on this "fake" pixel layer.