[Qgis-developer] Rejigging raster layer drawing

Barry Rowlingson b.rowlingson at lancaster.ac.uk
Fri Apr 27 13:54:48 EDT 2007

Peter Ersts wrote:

> Thing specifically being addressed at this time are:
> Stretching
> Transparency
> Color maps

  Here's what I've been thinking. A QgsRasterLayer is composed of a 
number of bands.  There's potentially a separate mapping for each band 
that converts band values to RGBA colours.  Only some bands are 'live'. 
Then the final colour is composed from the colours of the live bands.

  The mapping function can take into account stretching and scaling (and 
individually for each band) as well as colourmap rotation.

  This seems fully general (if slightly inefficent), since you can map 
single band layers to gray colour ramps, or red colour ramps, or complex 
palette-style colour ramps.

  Multi-band rasters can do just about anything. If you have four bands 
you could have red, blue, green and yellow ramps.  A fifth band could 
just set transparency.  You could just have one live band and do single 
band grayscale, or have three live bands and do RGB colour, or with 
three live bands each with grayscale ramps, get a full grayscale image.

  That last examples are where the inefficiency is - you're doing three 
colourmap lookups and then combining three colours rather than adding 
three numbers and doing one colourmap lookup - or with RGBA in four 
separate bands just constructing the colour from the band values with no 
lookups.  Perhaps there's a need for a more efficient rendering only 
when bands map directly to RGBA colours.

  Colourmap palettes could initially come from the dataset itself if it 
supplies one, or defined by the user (have you tried the Qt Gradient 

  A gui for this shouldn't be too bewildering, since there are plenty of 
opportunities for simple presets and then a big fat 'Advanced' button.

  Maybe I'm way off-beam on this, but in my mind, it should translate 
into quite simple and neat code...  But then most things that seem 
simple and neat in my mind never work out that way!


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