Color gradient lines based on their slopes

Sloppy gradient colored lines

-- NOTE :
-- This gist has two separate parts : one for the geometry generator, one for the filling color
-- inputs : lines, slope raster

-- STEP 1/2 - On geometry generator, set this expression to have interpolated points :

collect_geometries(
	array_foreach(
		generate_series(0, length($geometry), 5), -- we add lines every 5 meters
		line_interpolate_point($geometry, @element)
	)
)

-- STEP 2/2 - On filling color, set this expression :

with_variable(
  'min',
	raster_statistic('slope', 1, 'min'), -- global min value of slope raster

	with_variable(
    'max',
    raster_statistic('slope', 1, 'max'), -- global max value of slope raster

    with_variable(
      'start',
      scale_linear(
        raster_value(
          'slope',
          1,
          start_point($geometry) -- we get the raster slope value for the starting node
        ),
        @min, @max,
        0,1 -- color gradients are between 0 (bottom of the gradient) and 1 (top of the gradient)
       ),

    with_variable(
      'end',

    scale_linear(
      raster_value(
        'pente',
        1,
        end_point($geometry)  -- we get the raster slope value for the ending node
       ),
      @min, @max,
      0,1
     ),

    ramp_color( -- this function allows you to select one particular color along a color ramp
      'Viridis', -- I ❤ Viridis
      scale_linear(
      	@geometry_part_num, -- @geometry_part_num is the number of the point along the line (its position)
      	0, @geometry_part_count, -- @geometry_part_count is the number for points along the line
      	@start, -- start node gradient slope value (stored in a variable)
      	@end -- end node gradient slope value (stored in a variable)
      )
    )
   )
  )
 )
)