Load a raster layer in QGIS.

Add a new Scratch Layer of Polygon type (in any CRS). Set its Symbology to Inverted Polygons. Use a Geometry Generator as symbol layer type. Set it to LineString/MultiLineString. Enter the expression below and adjust the layer ID (best enter the expression editor and find it in the “layers” tree). Then adjust the scaling factor at the very bottom.

• 

• 

• 

• 

• 

• 

• 

• 

• 

• 

• 

-- UPPER CASE comments below are where you can change things

with_variable(
	'raster_layer',
	'long_and_complicated_layer_id',  -- RASTER LAYER to sample from
	-- this collects all the linestrings generated below into one multilinestring
	collect_geometries(
		-- a loop for each y value of the grid
		array_foreach(
			-- array_foreach loops over all elements of the series generated below
			-- which is a range of numbers from the bottom to the top of y values
			-- of the map canvas extent coordinates.
			-- the result will be an array of linestrings
			generate_series(
				y(@map_extent_center)-(@map_extent_height/2), -- bottom y
				y(@map_extent_center)+(@map_extent_height/2),  -- top y
				@map_extent_height/50  -- stepsize -> HOW MANY LINES
			),
			
			-- we want to enter another loop so we assign the name 'y' to
			-- the current element of the array_foreach loop
			with_variable(
				'y',
				@element,
				
				-- now we are ready to generate the line for this y value
				make_line(
					-- another loop, this time for the x values. same logic as before
					-- the result will be an array of points
					array_foreach(
						generate_series(
							x(@map_extent_center)-(@map_extent_width/2), -- left x
							x(@map_extent_center)+(@map_extent_width/2),  -- right x
							@map_extent_width/50  -- stepsize -> HOW MANY POINTS PER LINE
						),
						-- and here we create each point of the line
						make_point(
							@element,  -- the current value from the loop over the x value range
							@y  -- the y value from the outer loop
							+   -- will get an additional offset to generate the effect
							-- we look for values at _this point_ in the raster, and since
							-- the raster might not have any value here, we must use coalesce
							-- to use a replacement value in those cases
							coalesce(  -- coalesce to catch raster null values
								raster_value(
									@raster_layer,
									1,  -- band 1, *snore*
									-- to look up the raster value we need to look in the right position
									-- so we make a sampling point in the same CRS as the raster layer
									transform( 
										make_point(@element, @y),
										@map_crs,
										layer_property(@raster_layer,'crs')
									)
								),
								0  -- coalesce 0 if raster_value gave null
							-- here is where we set the scaling factor for the raster -> y values
							-- if things are weird, set it to 0 and try small multiplications or divisions
							-- to see what happens.
							-- for metric systems you will want to multiply
							-- for geographic coordinates you will want to divide
							)*10  -- user-defined factor for VERTICAL EXAGGERATION
						)
					)
				)
			)
		)
	)
)  -- wee

If you don’t have your raster data on a SSD this can be a bit slow.

Yes, this works if you change your CRS!

A slight misunderstanding about a weird pattern I posted to Twitter earlier made me wonder how easy it might be to present elevation data with real depth ™ as anaglyph 3D in QGIS. Anaglyph 3D, you know, those red and cyan glasses that rarely ever worked.

You might remember my post on fake chromatic aberration and dynamic label shadows in QGIS some months ago. Maybe with some small adjustments of the technique…?

Wikipedia has an detailed article on the topic if you are interested. It boils down to: One eye gets to see not the red but the cyan stuff. The other sees the red but not the cyan. By copying, coloring and shifting elements left-right increasingly the human vision gets tricked into seeing fake depth.

Got your anaglyph glasses? Oogled some images you found online to get in the mood? Excellent, let’s go!

Get some DEM data. I used a 10 meter DEM of Hamburg and a 200 meter DEM of Germany for testing. Make sure both your data and your project/canvas CRS match or you have a hard time with the math.

Extract contours at a reasonable interval. What’s reasonable depends on your data. I used an interval of 5 meters for the 10 meter DEM as Hamburg is rather flat (which you will NOT see here).

You may want to simplify or smooth this.

Now it’s time to zoom in, because the effect only works if the line density is appropriately light.

Now we need to duplicate those, color them and move them left and right.

Change the symbol layer of your contours to a Geometry Generator.

Let’s use that first layer for the left, red part. So change the color to red. Use a red that vanishes when you look through your left (red) eye but is clearly visible through the right (cyan) eye. The exact color depends on your glasses.

Set the Geometry Generator to LineString. I will now explain an intermediate step so if you just want the result, scroll a bit.

translate(
  $geometry,
  -  -- there is a minus here!
  (
    x_max(@map_extent) - x(centroid($geometry))
  )
  /100  -- magic scale value…
  ,
  0  -- no translation in y
)

This moves each contour line to the left for a value that increases with the geometry’s distance to the right side. Since we don’t want to move the geometries too far, a magic scale factor needs to be added and adjusted according to your coordinate values.

(Yes, that is a bug right here (centroid is a bad metric and some of the contours are huge geometries) but hey it works well enough. Segmentizing could fix this. Or just extract the vertices, that looks cool too TODO imagelink)

For the right, cyan side we need to add another Geometry Generator symbol layer, color it in cyan (so that you can only see it through your left eye) and do the geometry expression the other way around:

translate(
  $geometry,
  (
    x(centroid($geometry))-x_min(@map_extent)
  )
  /100  -- magic scale value…
  ,
  0  -- no translation in y
)

Cool! But this is lacking a certain depth, don’t you think? We need to scale the amount of horizontal shift by the elevation value of the contour lines. And then we also need to adjust the magic scale value again because reasons.

For the red symbol layer:

translate(
  $geometry,
  -- move to the LEFT
  -- scaled by the distance to the RIGHT side of the map
  -- scaled by the elevation

  -- minus so it goes to the left
  -
  "ELEV" -- the attribute field with the elevation values
  *
  (x_max(@map_extent)-x(centroid($geometry)))
  
  -- MAGIC scale value according to CRS and whatever
  /10000
  ,
  -- no change for y
  0
)

For the cyan symbol layer:

translate(
  $geometry,
  -- move to the RIGHT
  -- scaled by the distance to the LEFT side of the map
  -- scaled by the elevation
 
  "ELEV" -- the attribute field with the elevation values
  *
  (x(centroid($geometry))-x_min(@map_extent))
  
  -- MAGIC scale value according to CRS and whatever
  /10000
  ,
  -- no change for y
  0
)

That’s it!

Now, who is going to do Autostereograms? Also check out http://elasticterrain.xyz.