Choose terrain levels from what the screen can resolve

Levels changed at fixed distances - 512 m, 1 km, 2 km and on - whatever the window, the
field of view or the zoom. Now a level is used out to the distance where one of its sample
spacings covers terraindetail pixels on screen, 3 by default, which follows from the
viewport height and the vertical field of view with the zoom applied; each level doubles
the spacing, so each doubles the distance. Zooming in or a taller window pushes detail
further, a coarser setting pulls it in. The finest range never drops below four tile
sides, which the morphing needs to close the seams.

Levels and morphing both follow the finest range the last scan used, so they always agree,
and a change of more than a percent - a zoom, a resize - triggers a rescan.
tools/terraincook/seamcheck.py now also tries finest ranges of 5.7 and 11.3 tiles, and
finds no gap in any of them.

terraindetail is in the ini and on a slider in the debug panel; the panel's level table
and the residency line in the log show the resulting ranges. On baltyk the current window
works out to 484 m, so the 512 m minimum applies and nothing changes by default; a
scenery setting terraindetail 1 got 1349 m, and the samples on the gpu went from 3 MB to
10 MB with it.

application/driveruipanels.cpp

@@ -773,6 +773,8 @@ debug_panel::render_section_scenario() {
 				(to_string(std::exp(terrainrange), 0, 5) + " m###terrainrange").c_str(), &terrainrange, std::log(500.0f), std::log(100000.0f), "Terrain range")) {
 				Global.TerrainRange = std::clamp(std::exp(terrainrange), 500.0f, 100000.0f);
 			}
+			// pixels a terrain sample may cover before a finer level takes over
+			ImGui::SliderFloat("###terraindetail", &Global.TerrainDetail, 1.0f, 8.0f, "Terrain detail: %.1f px per sample");
 		}
     }
 
@@ -1524,8 +1526,8 @@ debug_panel::update_section_renderer( std::vector<text_line> &Output ) {
                     auto const last { level + 1 == field.levels };
                     levels +=
                         " L" + std::to_string( level )
-                        + ( last ? " beyond " + to_string( terrain_level_distance( level > 0 ? level - 1 : 0, field.tilesize ), 0 )
-                                 : " <" + to_string( terrain_level_distance( level, field.tilesize ), 0 ) )
+                        + ( last ? " beyond " + to_string( terrain_level_distance( level > 0 ? level - 1 : 0, field.finest ), 0 )
+                                 : " <" + to_string( terrain_level_distance( level, field.finest ), 0 ) )
                         + " m (" + to_string( field.gridstep * static_cast<float>( 1u << level ), 0 ) + " m grid): "
                         + std::to_string( stats.perlevel[ level ] );
                     if( false == last ) { levels += " |"; }

rendering/opengl33renderer.cpp

@@ -2912,6 +2912,11 @@ void opengl33_renderer::Render_terrain()
 		{
 			// the terrain has a range of its own, not the pass draw range
 			field->range(Global.TerrainRange);
+			// how far each level reaches follows from what the view can resolve
+			auto const fovy{glm::radians(Global.FieldOfView / Global.ZoomFactor)};
+			field->detail(
+			    static_cast<double>(m_current_viewport->height) / (2.0 * std::tan(fovy * 0.5)),
+			    Global.TerrainDetail);
 			field->update(viewpoint);
 		}
 		field->render(viewpoint);
@@ -2932,6 +2937,7 @@ void opengl33_renderer::Render_terrain()
 				status.levels = field.levels();
 			}
 			status.range = field.range();
+			status.finest = field.finest();
 			status.backlog = field.backlog();
 			status.stats = field.stats();
 		}

rendering/terrainclipmap.cpp

@@ -208,9 +208,8 @@ terrain_clipmap::build_palette() {
 std::uint32_t
 terrain_clipmap::level_for( double const Distance ) const {
 
-    auto const finest { terrain_level_distance( 0, m_reader.tilesize() ) };
-    if( Distance <= finest ) { return 0; }
-    auto const steps { static_cast<std::uint32_t>( std::log2( Distance / finest ) ) + 1 };
+    if( Distance <= m_finest ) { return 0; }
+    auto const steps { static_cast<std::uint32_t>( std::log2( Distance / m_finest ) ) + 1 };
     return std::min( steps, m_reader.levels() - 1 );
 }
 
@@ -265,11 +264,18 @@ terrain_clipmap::retire( resident_tile &Tile ) {
     m_stats.texturebytes -= std::min( m_stats.texturebytes, bytes );
 }
 
+void
+terrain_clipmap::detail( double const Pixelsperunit, double const Pixels ) {
+
+    m_finestwanted = terrain_finest_range( m_reader.gridstep(), m_reader.tilesize(), Pixelsperunit, Pixels );
+}
+
 void
 terrain_clipmap::scan( glm::dvec3 const &Viewpoint ) {
 
     m_scanpoint = Viewpoint;
     m_scanrange = m_range;
+    m_finest = ( m_finestwanted > 0.0 ? m_finestwanted : terrain_finest_tiles * m_reader.tilesize() );
     m_inrange.clear();
     m_wantedlevel.clear();
 
@@ -324,7 +330,12 @@ terrain_clipmap::update( glm::dvec3 const &Viewpoint ) {
 
     auto const moved {
         glm::length( glm::dvec2 { Viewpoint.x - m_scanpoint.x, Viewpoint.z - m_scanpoint.z } ) };
+    // a zoom or a resized window changes the levels; a change under a percent is not worth
+    // redoing the scan for
+    auto const detailchanged {
+        ( m_finestwanted > 0.0 ) && ( std::abs( m_finestwanted - m_finest ) > 0.01 * m_finest ) };
     if( ( m_range != m_scanrange )
+     || ( true == detailchanged )
      || ( moved > m_reader.tilesize() * terrain_rescan_tiles ) ) {
         scan( Viewpoint );
     }
@@ -446,7 +457,8 @@ terrain_clipmap::report_residency() {
     WriteLog(
         "Terrain: " + std::to_string( m_stats.resident ) + " tiles resident, "
         + std::to_string( m_stats.texturebytes / 1048576 ) + " MB of samples, "
-        + std::to_string( m_stats.uploaded ) + " uploads so far" );
+        + std::to_string( m_stats.uploaded ) + " uploads so far, finest level out to "
+        + std::to_string( static_cast<int>( m_finest ) ) + " m" );
 }
 
 void
@@ -487,8 +499,8 @@ terrain_clipmap::render( glm::dvec3 const &Viewpoint ) {
         if( tile.level + 1 < m_reader.levels() ) {
             ::glUniform2f(
                 m_uniforms.morph,
-                static_cast<float>( terrain_morph_begin( tile.level, tilesize ) ),
-                static_cast<float>( terrain_morph_end( tile.level, tilesize ) ) );
+                static_cast<float>( terrain_morph_begin( tile.level, m_finest, tilesize ) ),
+                static_cast<float>( terrain_morph_end( tile.level, m_finest, tilesize ) ) );
         }
         else {
             // far enough to never be reached, with a band that does not divide by zero

rendering/terrainclipmap.cppm

@@ -71,6 +71,11 @@ public:
     // how far from the viewpoint terrain is drawn, in metres
     void range( float const Range ) { m_range = Range; }
     float range() const { return m_range; }
+    // what the screen can resolve: the viewport's pixels per unit of size over distance, and
+    // how many pixels a sample spacing may cover. decides how far each level reaches
+    void detail( double const Pixelsperunit, double const Pixels );
+    // the finest level's range the tiles in range were last worked out with
+    float finest() const { return static_cast<float>( m_finest ); }
 
     // metres between samples at the finest level; a scenery's fields are drawn from the
     // finest to the coarsest so that the depth test keeps the better surface
@@ -135,6 +140,10 @@ private:
     std::unordered_map<std::int64_t, std::uint32_t> m_wantedlevel;
     glm::dvec3 m_scanpoint { 0.0 };
     float m_scanrange { -1.f };
+    // the finest level's range the screen asks for, and the one the last scan used. levels and
+    // morphing both follow the scanned one, so they always agree
+    double m_finestwanted { 0.0 };
+    double m_finest { 0.0 };
     std::unordered_map<std::int64_t, resident_tile> m_tiles;
     // one index buffer per mip level, over a grid of that level's sample count. vertices
     // are computed in the shader from gl_VertexID, so there is no vertex buffer at all.

rendering/terrainstatus.cppm

@@ -8,6 +8,7 @@ http://mozilla.org/MPL/2.0/.
 */
 
 module;
+#include <algorithm>
 #include <array>
 #include <cmath>
 #include <cstddef>
@@ -19,20 +20,29 @@ export module eu07.rendering.terrainstatus;
 
 export {
 
-// distance out to which a terrain tile is drawn at its finest level, in tile sides; each
-// further level doubles it. four sides is 512 m for the usual 128 m tile. the ranges are
-// measured in tiles rather than metres because the morphing below only closes the seams
-// when a level's range is several tiles long, whatever the tile size
+// Which level a tile is drawn at follows from what its samples look like on screen: a level is
+// used out to the distance where one of its sample spacings covers the detail setting in
+// pixels. Each level doubles the spacing, so each doubles that distance too. Pixelsperunit is
+// the viewport height over twice the tangent of half the vertical field of view - what a
+// metre at a metre's distance spans - so zooming in or a taller window pushes detail further.
+//
+// The finest range never drops below four tile sides, whatever the screen: the morphing below
+// only closes the seams when a level's range is several tiles long.
 inline constexpr double terrain_finest_tiles { 4.0 };
 
+inline double
+terrain_finest_range( double const Gridstep, double const Tilesize, double const Pixelsperunit, double const Detail ) {
+    return std::max( terrain_finest_tiles * Tilesize, Gridstep * Pixelsperunit / std::max( 0.1, Detail ) );
+}
+
 // how far the camera may move before the tiles in range and their levels are worked out
 // again, in tile sides
 inline constexpr double terrain_rescan_tiles { 0.25 };
 
-// the distance up to which a level is used, for tiles of the given side
+// the distance up to which a level is used, given the finest level's range
 inline double
-terrain_level_distance( std::uint32_t const Level, double const Tilesize ) {
-    return terrain_finest_tiles * Tilesize * std::pow( 2.0, Level );
+terrain_level_distance( std::uint32_t const Level, double const Finest ) {
+    return Finest * std::pow( 2.0, Level );
 }
 
 // A tile morphs into the next level over a band at the far end of its level's range. Two
@@ -42,19 +52,20 @@ terrain_level_distance( std::uint32_t const Level, double const Tilesize ) {
 // was at the last rescan while the morph follows the camera every frame; and it is a fifth
 // of the range wide, which keeps the coarser tile's band, a whole range further out, clear of
 // any edge a finer tile can reach. Both were settled by a geometric test over random camera
-// positions and three tile sizes, which finds no gap with these values and finds them at
-// every level boundary with a band ending at the range itself.
+// positions, three tile sizes and several finest ranges (tools/terraincook/seamcheck.py),
+// which finds no gap with these values and finds them at every level boundary with a band
+// ending at the range itself.
 inline constexpr double terrain_morph_band { 0.2 };
 
 // the distances, from the camera, over which a level L tile morphs into level L+1
 inline double
-terrain_morph_end( std::uint32_t const Level, double const Tilesize ) {
-    return terrain_level_distance( Level, Tilesize ) - terrain_rescan_tiles * Tilesize;
+terrain_morph_end( std::uint32_t const Level, double const Finest, double const Tilesize ) {
+    return terrain_level_distance( Level, Finest ) - terrain_rescan_tiles * Tilesize;
 }
 
 inline double
-terrain_morph_begin( std::uint32_t const Level, double const Tilesize ) {
-    return terrain_morph_end( Level, Tilesize ) - terrain_morph_band * terrain_level_distance( Level, Tilesize );
+terrain_morph_begin( std::uint32_t const Level, double const Finest, double const Tilesize ) {
+    return terrain_morph_end( Level, Finest, Tilesize ) - terrain_morph_band * terrain_level_distance( Level, Finest );
 }
 
 // what one terrain field's streamer is doing, kept by the field and read by the debug panel
@@ -75,6 +86,7 @@ struct terrain_field_status {
     float gridstep { 0.f };
     float tilesize { 0.f };
     float range { 0.f };
+    float finest { 0.f };           // range of the finest level, from the screen
     std::uint32_t levels { 0 };
     std::size_t backlog { 0 };      // tiles the loader has queued or finished and not handed over
     terrain_statistics stats;

tools/terraincook/seamcheck.py

@@ -16,14 +16,16 @@ import math
 import random
 import sys
 
-FINEST_TILES = 4.0   # terrain_finest_tiles
+# finest ranges tried, in tile sides: terrain_finest_tiles is the minimum, and the screen
+# can push it further (terrain_finest_range)
+FINEST_TILES = (4.0, 5.7, 11.3)
 RESCAN_TILES = 0.25  # terrain_rescan_tiles
 MORPH_BAND = 0.2     # terrain_morph_band
 LEVELS = 6
 
 
-def check(tilesize, step, trials, reach):
-    finest = FINEST_TILES * tilesize
+def check(tilesize, step, trials, reach, finest_tiles):
+    finest = finest_tiles * tilesize
 
     def level_range(level):
         return finest * 2 ** level
@@ -84,10 +86,12 @@ def check(tilesize, step, trials, reach):
 def main():
     random.seed(1)
     failed = False
-    for tilesize, step, trials, reach in ((128, 1.0, 300, 40), (256, 2.0, 300, 40), (12800, 100.0, 60, 6)):
-        gaps, example = check(tilesize, step, trials, reach)
-        print(f"tile {tilesize} m, grid {step} m: {gaps} gaps" + (f" (e.g. {example})" if gaps else ""))
-        failed = failed or gaps > 0
+    for finest_tiles in FINEST_TILES:
+        for tilesize, step, trials, reach in ((128, 1.0, 150, 60), (256, 2.0, 150, 60), (12800, 100.0, 30, 8)):
+            gaps, example = check(tilesize, step, trials, reach, finest_tiles)
+            print(f"finest {finest_tiles} tiles, tile {tilesize} m, grid {step} m: {gaps} gaps"
+                  + (f" (e.g. {example})" if gaps else ""))
+            failed = failed or gaps > 0
     return 1 if failed else 0
 
 

utilities/Globals.cpp

@@ -271,6 +271,12 @@ bool global_settings::ConfigParseGraphics(cParser& Parser, const std::string& to
         return true;
     }
 
+    if (token == "terraindetail")
+    {
+        ParseOne(Parser, TerrainDetail, 1);
+        return true;
+    }
+
     if (token == "vsync")
     {
         ParseOne(Parser, VSync);
@@ -1540,6 +1546,7 @@ global_settings::export_as_text( std::ostream &Output ) const {
     export_as_text( Output, "targetfps", targetfps );
     export_as_text( Output, "basedrawrange", BaseDrawRange );
     export_as_text( Output, "terrainrange", TerrainRange );
+    export_as_text( Output, "terraindetail", TerrainDetail );
     export_as_text( Output, "fullscreen", bFullScreen );
     export_as_text( Output, "fullscreenmonitor", fullscreen_monitor );
     export_as_text( Output, "fullscreenwindowed", fullscreen_windowed );

utilities/Globals.cppm

@@ -196,6 +196,9 @@ struct global_settings {
     // how far terrain tiles are streamed and drawn, independent of BaseDrawRange: a baked
     // heightfield is cheap far out, and seeing it to the horizon is the point of having it
     float TerrainRange{ 20000.f };
+    // pixels one terrain sample spacing may cover on screen before a finer level takes over.
+    // smaller is sharper and costs more tiles at their finest
+    float TerrainDetail{ 3.f };
     int DynamicLightCount{ 7 };
     bool ScaleSpecularValues{ true };
     std::string GfxRenderer{ "default" };