DOC: Adding a section about default pixel functions in vrt tutorial (…

…from PR review)

gdal/frmts/vrt/vrt_tutorial.dox

@@ -806,6 +806,30 @@ calling the pixel function, and the imaginary portion would be lost.
     ...
 \endcode
 
+<h3> Default Pixel Functions </h3>
+
+GDAL provides a set of default pixel functions that can be used without writing new code:
+
+<ul>
+<li><b>"real": </b> extract real part from a single raster band (just a copy if the input is non-complex)
+<li><b>"imag": </b>
+    extract imaginary part from a single raster band (0 for non-complex)
+<li><b> "complex":   </b> make a complex band merging two bands used as real and imag values
+<li><b> "mod":       </b> extract module from a single raster band (real or complex)
+<li><b> "phase":     </b> extract phase from a single raster band (0 for non-complex)
+<li><b> "conj":      </b> computes the complex conjugate of a single raster band (just a copy if the input is non-complex)
+<li><b> "sum":       </b> sum 2 or more raster bands
+<li><b> "diff":      </b> computes the difference between 2 raster bands (b1 - b2)
+<li><b> "mul":       </b> multiply 2 or more raster bands
+<li><b> "cmul":      </b> multiply the first band for the complex conjugate of the second
+<li><b> "inv":       </b>  inverse (1./x). Note: no check is performed on zero division
+<li><b> "intensity": </b> computes the intensity Re(x*conj(x)) of a single raster band (real or complex)
+<li><b> "sqrt":      </b> perform the square root of a single raster band (real only)
+<li><b> "log10":     </b> compute the logarithm (base 10) of the abs of a single raster band (real or complex): log10( abs( x ) )
+<li><b> "dB2amp":    </b> perform scale conversion from logarithmic to linear (amplitude) (i.e. 10 ^ ( x / 20 ) ) of a single raster band (real only)
+<li><b> "dB2pow":    </b> perform scale conversion from logarithmic to linear (power) (i.e. 10 ^ ( x / 10 ) ) of a single raster band (real only)
+</ul>
+
 <h3>Writing Pixel Functions</h3>
 
 To register this function with GDAL (prior to accessing any VRT datasets