[core] better symbol fading with texture lookups (#4579)

- this is simpler than predicting opacity based on current zooming speed
- this is smoother: symbols don't flicker when changing zoom speed or
  when zooming in and out rapidly.

https://github.com/mapbox/mapbox-gl-js/commit/1df146627092b58c8db64d45530c74cd12a3fa02

fix #4562

10 files changed, 135 insertions, 162 deletions

diff --git a/src/mbgl/renderer/frame_history.cpp b/src/mbgl/renderer/frame_history.cpp
index bf339edc6f..eb125201f4 100644
--- a/src/mbgl/renderer/frame_history.cpp
+++ b/src/mbgl/renderer/frame_history.cpp
@@ -3,82 +3,110 @@
 
 using namespace mbgl;
 
-// Record frame history that will be used to calculate fading params
-void FrameHistory::record(const TimePoint& now, float zoom) {
-    // first frame ever
-    if (history.empty()) {
-        history.emplace_back(FrameSnapshot{TimePoint::min(), zoom});
-        history.emplace_back(FrameSnapshot{TimePoint::min(), zoom});
-    }
+FrameHistory::FrameHistory() {
+    changeTimes.fill(TimePoint::min());
+    changeOpacities.fill(0);
+    opacities.fill(0);
+};
 
-    if (!history.empty() || history.back().z != zoom) {
-        history.emplace_back(FrameSnapshot{now, zoom});
-    }
-}
+void FrameHistory::record(const TimePoint& now, float zoom, const Duration& duration) {
 
-bool FrameHistory::needsAnimation(const Duration& duration) const {
-    if (history.empty()) {
-        return false;
-    }
+    int16_t zoomIndex = std::floor(zoom * 10.0);
 
-    // If we have a value that is older than duration and whose z value is the
-    // same as the most current z value, and if all values inbetween have the
-    // same z value, we don't need animation, otherwise we probably do.
-    const FrameSnapshot& pivot = history.back();
+    if (firstFrame) {
 
-    int i = -1;
-    while ((int)history.size() > i + 1 && history[i + 1].now + duration < pivot.now) {
-        ++i;
+        for (int16_t z = 0; z <= zoomIndex; z++) {
+            opacities[z] = 255u;
+        }
+        firstFrame = false;
     }
 
-    if (i < 0) {
-        // There is no frame that is older than the duration time, so we need to
-        // check all frames.
-        i = 0;
+    if (zoomIndex < previousZoomIndex) {
+        for (int16_t z = zoomIndex + 1; z <= previousZoomIndex; z++) {
+            changeTimes[z] = now;
+            changeOpacities[z] = opacities[z];
+        }
+    } else {
+        for (int16_t z = zoomIndex; z > previousZoomIndex; z--) {
+            changeTimes[z] = now;
+            changeOpacities[z] = opacities[z];
+        }
     }
 
-    // Make sure that all subsequent snapshots have the same zoom as the last
-    // pivot element.
-    for (; (int)history.size() > i; ++i) {
-        if (history[i].z != pivot.z) {
-            return true;
+    for (int16_t z = 0; z <= 255; z++) {
+        std::chrono::duration<float> timeDiff = now - changeTimes[z];
+        int32_t opacityChange = (duration == Milliseconds(0) ? 1 : (timeDiff / duration)) * 255;
+        if (z <= zoomIndex) {
+            opacities[z] = util::min(255, changeOpacities[z] + opacityChange);
+        } else {
+            opacities[z] = util::max(0, changeOpacities[z] - opacityChange);
         }
     }
 
-    return false;
+    changed = true;
+
+    if (zoomIndex != previousZoomIndex) {
+        previousZoomIndex = zoomIndex;
+        previousTime = now;
+    }
+
+    time = now;
 }
 
-FadeProperties FrameHistory::getFadeProperties(const TimePoint& now, const Duration& duration) {
-    // Remove frames until only one is outside the duration, or until there are only three
-    while (history.size() > 3 && history[1].now + duration < now) {
-        history.pop_front();
+bool FrameHistory::needsAnimation(const Duration& duration) const {
+    return (time - previousTime) < duration;
+}
+
+void FrameHistory::upload(gl::GLObjectStore& glObjectStore) {
+
+    if (changed) {
+        const bool first = !texture;
+        bind(glObjectStore);
+
+        if (first) {
+            MBGL_CHECK_ERROR(glTexImage2D(
+                        GL_TEXTURE_2D, // GLenum target
+                        0, // GLint level
+                        GL_ALPHA, // GLint internalformat
+                        width, // GLsizei width
+                        height, // GLsizei height
+                        0, // GLint border
+                        GL_ALPHA, // GLenum format
+                        GL_UNSIGNED_BYTE, // GLenum type
+                        opacities.data()
+                        ));
+        } else {
+            MBGL_CHECK_ERROR(glTexSubImage2D(
+                        GL_TEXTURE_2D, // GLenum target
+                        0, // GLint level
+                        0, // GLint xoffset
+                        0, // GLint yoffset
+                        width, // GLsizei width
+                        height, // GLsizei height
+                        GL_ALPHA, // GLenum format
+                        GL_UNSIGNED_BYTE, // GLenum type
+                        opacities.data()
+                        ));
+        }
+
+        changed = false;
+
     }
+}
 
-    if (history[1].now + duration < now) {
-        history[0].z = history[1].z;
+void FrameHistory::bind(gl::GLObjectStore& glObjectStore) {
+    if (!texture) {
+        texture.create(glObjectStore);
+        MBGL_CHECK_ERROR(glBindTexture(GL_TEXTURE_2D, texture.getID()));
+#ifndef GL_ES_VERSION_2_0
+        MBGL_CHECK_ERROR(glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAX_LEVEL, 0));
+#endif
+        MBGL_CHECK_ERROR(glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE));
+        MBGL_CHECK_ERROR(glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE));
+        MBGL_CHECK_ERROR(glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST));
+        MBGL_CHECK_ERROR(glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST));
+    } else {
+        MBGL_CHECK_ERROR(glBindTexture(GL_TEXTURE_2D, texture.getID()));
     }
 
-    // Find the range of zoom levels we want to fade between
-    float startingZ = history.front().z;
-    const FrameSnapshot lastFrame = history.back();
-    float endingZ = lastFrame.z;
-    float lowZ = util::min(startingZ, endingZ);
-    float highZ = util::max(startingZ, endingZ);
-
-    // Calculate the speed of zooming, and how far it would zoom in terms of zoom levels in one
-    // duration
-    float zoomDiff = endingZ - history[1].z;
-    std::chrono::duration<float> timeDiff = lastFrame.now - history[1].now;
-    float fadedist = zoomDiff / (timeDiff / duration);
-
-    // At end of a zoom when the zoom stops changing continue pretending to zoom at that speed
-    // bump is how much farther it would have been if it had continued zooming at the same rate
-    float bump = std::chrono::duration<float>(now - lastFrame.now) / duration * fadedist;
-
-    return FadeProperties {
-        fadedist,
-        lowZ,
-        highZ,
-        bump
-    };
 }
diff --git a/src/mbgl/renderer/frame_history.hpp b/src/mbgl/renderer/frame_history.hpp
index 13a953e681..88c58d74a0 100644
--- a/src/mbgl/renderer/frame_history.hpp
+++ b/src/mbgl/renderer/frame_history.hpp
@@ -1,37 +1,38 @@
 #ifndef MBGL_RENDERER_FRAME_HISTORY
 #define MBGL_RENDERER_FRAME_HISTORY
 
-#include <deque>
-#include <cassert>
-#include <cmath>
+#include <array>
 
 #include <mbgl/platform/platform.hpp>
+#include <mbgl/gl/gl_object_store.hpp>
 #include <mbgl/util/chrono.hpp>
 
 namespace mbgl {
 
-struct FrameSnapshot {
-    const TimePoint now;
-    float z;
-};
-
-struct FadeProperties {
-    float fadedist;
-    float minfadezoom;
-    float maxfadezoom;
-    float bump;
-};
-
 class FrameHistory {
 public:
-    // Record frame history that will be used to calculate fading params
-    void record(const TimePoint&, float zoom);
+    FrameHistory();
+    void record(const TimePoint&, float zoom, const Duration&);
 
     bool needsAnimation(const Duration&) const;
-    FadeProperties getFadeProperties(const TimePoint&, const Duration&);
+    void bind(gl::GLObjectStore&);
+    void upload(gl::GLObjectStore&);
 
 private:
-    std::deque<FrameSnapshot> history;
+    const int width = 256;
+    const int height = 1;
+
+    std::array<TimePoint, 256> changeTimes;
+    std::array<uint8_t, 256> changeOpacities;
+    std::array<uint8_t, 256> opacities;
+
+    int16_t previousZoomIndex = 0;
+    TimePoint previousTime = TimePoint::min();
+    TimePoint time = TimePoint::min();
+    bool firstFrame = true;
+    bool changed = true;
+
+    gl::TextureHolder texture;
 };
 
 } // namespace mbgl
diff --git a/src/mbgl/renderer/painter.cpp b/src/mbgl/renderer/painter.cpp
index e993e3dddf..175770dbb1 100644
--- a/src/mbgl/renderer/painter.cpp
+++ b/src/mbgl/renderer/painter.cpp
@@ -107,6 +107,9 @@ void Painter::render(const Style& style, const FrameData& frame_, SpriteAtlas& a
     matrix::identity(nativeMatrix);
     matrix::multiply(nativeMatrix, projMatrix, nativeMatrix);
 
+    frameHistory.record(frame.timePoint, state.getZoom(),
+        frame.mapMode == MapMode::Continuous ? util::DEFAULT_FADE_DURATION : Milliseconds(0));
+
     // - UPLOAD PASS -------------------------------------------------------------------------------
     // Uploads all required buffers and images before we do any actual rendering.
     {
@@ -117,6 +120,7 @@ void Painter::render(const Style& style, const FrameData& frame_, SpriteAtlas& a
         spriteAtlas->upload(glObjectStore);
         lineAtlas->upload(glObjectStore);
         glyphAtlas->upload(glObjectStore);
+        frameHistory.upload(glObjectStore);
         annotationSpriteAtlas.upload(glObjectStore);
 
         for (const auto& item : order) {
@@ -161,8 +165,6 @@ void Painter::render(const Style& style, const FrameData& frame_, SpriteAtlas& a
         drawClippingMasks(generator.getStencils());
     }
 
-    frameHistory.record(frame.timePoint, state.getZoom());
-
     // Actually render the layers
     if (debug::renderTree) { Log::Info(Event::Render, "{"); indent++; }
 
diff --git a/src/mbgl/renderer/painter_symbol.cpp b/src/mbgl/renderer/painter_symbol.cpp
index 42355493a5..611f0c53e7 100644
--- a/src/mbgl/renderer/painter_symbol.cpp
+++ b/src/mbgl/renderer/painter_symbol.cpp
@@ -71,18 +71,9 @@ void Painter::renderSDF(SymbolBucket &bucket,
 
     sdfShader.u_zoom = (state.getZoom() - zoomAdjust) * 10; // current zoom level
 
-    if (frame.mapMode == MapMode::Continuous) {
-        FadeProperties f = frameHistory.getFadeProperties(frame.timePoint, util::DEFAULT_FADE_DURATION);
-        sdfShader.u_fadedist = f.fadedist * 10;
-        sdfShader.u_minfadezoom = std::floor(f.minfadezoom * 10);
-        sdfShader.u_maxfadezoom = std::floor(f.maxfadezoom * 10);
-        sdfShader.u_fadezoom = (state.getZoom() + f.bump) * 10;
-    } else { // MapMode::Still
-        sdfShader.u_fadedist = 0;
-        sdfShader.u_minfadezoom = state.getZoom() * 10;
-        sdfShader.u_maxfadezoom = state.getZoom() * 10;
-        sdfShader.u_fadezoom = state.getZoom() * 10;
-    }
+    config.activeTexture = GL_TEXTURE1;
+    frameHistory.bind(glObjectStore);
+    sdfShader.u_fadetexture = 1;
 
     // The default gamma value has to be adjust for the current pixelratio so that we're not
     // drawing blurry font on retina screens.
@@ -239,14 +230,13 @@ void Painter::renderSymbol(SymbolBucket& bucket,
 
             // adjust min/max zooms for variable font sies
             float zoomAdjust = std::log(fontSize / layout.iconSize) / std::log(2);
-
             iconShader->u_zoom = (state.getZoom() - zoomAdjust) * 10; // current zoom level
-            iconShader->u_fadedist = 0 * 10;
-            iconShader->u_minfadezoom = state.getZoom() * 10;
-            iconShader->u_maxfadezoom = state.getZoom() * 10;
-            iconShader->u_fadezoom = state.getZoom() * 10;
             iconShader->u_opacity = paint.iconOpacity;
 
+            config.activeTexture = GL_TEXTURE1;
+            frameHistory.bind(glObjectStore);
+            iconShader->u_fadetexture = 1;
+
             setDepthSublayer(0);
             bucket.drawIcons(*iconShader, glObjectStore);
         }
diff --git a/src/mbgl/shader/icon.fragment.glsl b/src/mbgl/shader/icon.fragment.glsl
index 45b56793eb..f9ed6e75ed 100644
--- a/src/mbgl/shader/icon.fragment.glsl
+++ b/src/mbgl/shader/icon.fragment.glsl
@@ -1,8 +1,11 @@
 uniform sampler2D u_texture;
+uniform sampler2D u_fadetexture;
+uniform float u_opacity;
 
 varying vec2 v_tex;
-varying float v_alpha;
+varying vec2 v_fade_tex;
 
 void main() {
-    gl_FragColor = texture2D(u_texture, v_tex) * v_alpha;
+    float alpha = texture2D(u_fadetexture, v_fade_tex).a * u_opacity;
+    gl_FragColor = texture2D(u_texture, v_tex) * alpha;
 }
diff --git a/src/mbgl/shader/icon.vertex.glsl b/src/mbgl/shader/icon.vertex.glsl
index 3850e01ab7..a6a423127a 100644
--- a/src/mbgl/shader/icon.vertex.glsl
+++ b/src/mbgl/shader/icon.vertex.glsl
@@ -9,18 +9,13 @@ attribute vec4 a_data2;
 uniform mat4 u_matrix;
 uniform mat4 u_exmatrix;
 uniform float u_zoom;
-uniform float u_fadedist;
-uniform float u_minfadezoom;
-uniform float u_maxfadezoom;
-uniform float u_fadezoom;
-uniform float u_opacity;
 uniform bool u_skewed;
 uniform float u_extra;
 
 uniform vec2 u_texsize;
 
 varying vec2 v_tex;
-varying float v_alpha;
+varying vec2 v_fade_tex;
 
 void main() {
     vec2 a_tex = a_data1.xy;
@@ -30,29 +25,9 @@ void main() {
     float a_minzoom = a_zoom[0];
     float a_maxzoom = a_zoom[1];
 
-    float a_fadedist = 10.0;
-
     // u_zoom is the current zoom level adjusted for the change in font size
     float z = 2.0 - step(a_minzoom, u_zoom) - (1.0 - step(a_maxzoom, u_zoom));
 
-    // fade out labels
-    float alpha = clamp((u_fadezoom - a_labelminzoom) / u_fadedist, 0.0, 1.0);
-
-    if (u_fadedist >= 0.0) {
-        v_alpha = alpha;
-    } else {
-        v_alpha = 1.0 - alpha;
-    }
-    if (u_maxfadezoom < a_labelminzoom) {
-        v_alpha = 0.0;
-    }
-    if (u_minfadezoom >= a_labelminzoom) {
-        v_alpha = 1.0;
-    }
-
-    // if label has been faded out, clip it
-    z += step(v_alpha, 0.0);
-
     if (u_skewed) {
         vec4 extrude = u_exmatrix * vec4(a_offset / 64.0, 0, 0);
         gl_Position = u_matrix * vec4(a_pos + extrude.xy, 0, 1);
@@ -63,6 +38,5 @@ void main() {
     }
 
     v_tex = a_tex / u_texsize;
-
-    v_alpha *= u_opacity;
+    v_fade_tex = vec2(a_labelminzoom / 255.0, 0.0);
 }
diff --git a/src/mbgl/shader/icon_shader.hpp b/src/mbgl/shader/icon_shader.hpp
index ec6311ea80..a7dcfe7a7e 100644
--- a/src/mbgl/shader/icon_shader.hpp
+++ b/src/mbgl/shader/icon_shader.hpp
@@ -15,15 +15,12 @@ public:
     UniformMatrix<4>                u_matrix      = {"u_matrix",      *this};
     UniformMatrix<4>                u_exmatrix    = {"u_exmatrix",    *this};
     Uniform<GLfloat>                u_zoom        = {"u_zoom",        *this};
-    Uniform<GLfloat>                u_fadedist    = {"u_fadedist",    *this};
-    Uniform<GLfloat>                u_minfadezoom = {"u_minfadezoom", *this};
-    Uniform<GLfloat>                u_maxfadezoom = {"u_maxfadezoom", *this};
-    Uniform<GLfloat>                u_fadezoom    = {"u_fadezoom",    *this};
     Uniform<GLfloat>                u_opacity     = {"u_opacity",     *this};
     Uniform<std::array<GLfloat, 2>> u_texsize     = {"u_texsize",     *this};
     Uniform<GLint>                  u_skewed      = {"u_skewed",      *this};
     Uniform<GLfloat>                u_extra       = {"u_extra",       *this};
     Uniform<GLint>                  u_texture     = {"u_texture",     *this};
+    Uniform<GLint>                  u_fadetexture = {"u_fadetexture", *this};
 
 protected:
     GLint a_offset = -1;
diff --git a/src/mbgl/shader/sdf.fragment.glsl b/src/mbgl/shader/sdf.fragment.glsl
index a77e959e53..3a6a4a8c37 100644
--- a/src/mbgl/shader/sdf.fragment.glsl
+++ b/src/mbgl/shader/sdf.fragment.glsl
@@ -1,15 +1,17 @@
 uniform sampler2D u_texture;
+uniform sampler2D u_fadetexture;
 uniform vec4 u_color;
 uniform float u_buffer;
 uniform float u_gamma;
 
 varying vec2 v_tex;
-varying float v_alpha;
+varying vec2 v_fade_tex;
 varying float v_gamma_scale;
 
 void main() {
     float dist = texture2D(u_texture, v_tex).a;
+    float fade_alpha = texture2D(u_fadetexture, v_fade_tex).a;
     float gamma = u_gamma * v_gamma_scale;
-    float alpha = smoothstep(u_buffer - gamma, u_buffer + gamma, dist) * v_alpha;
+    float alpha = smoothstep(u_buffer - gamma, u_buffer + gamma, dist) * fade_alpha;
     gl_FragColor = u_color * alpha;
 }
diff --git a/src/mbgl/shader/sdf.vertex.glsl b/src/mbgl/shader/sdf.vertex.glsl
index 0d538b24fd..818378475e 100644
--- a/src/mbgl/shader/sdf.vertex.glsl
+++ b/src/mbgl/shader/sdf.vertex.glsl
@@ -9,16 +9,12 @@ attribute vec4 a_data2;
 uniform mat4 u_matrix;
 uniform mat4 u_exmatrix;
 uniform float u_zoom;
-uniform float u_fadedist;
-uniform float u_minfadezoom;
-uniform float u_maxfadezoom;
-uniform float u_fadezoom;
 uniform bool u_skewed;
 
 uniform vec2 u_texsize;
 
 varying vec2 v_tex;
-varying float v_alpha;
+varying vec2 v_fade_tex;
 varying float v_gamma_scale;
 
 void main() {
@@ -32,24 +28,6 @@ void main() {
     // u_zoom is the current zoom level adjusted for the change in font size
     float show = step(a_minzoom, u_zoom) * (1.0 - step(a_maxzoom, u_zoom));
 
-    // fade out labels
-    float alpha = clamp((u_fadezoom - a_labelminzoom) / u_fadedist, 0.0, 1.0);
-
-    if (u_fadedist >= 0.0) {
-        v_alpha = alpha;
-    } else {
-        v_alpha = 1.0 - alpha;
-    }
-    if (u_maxfadezoom < a_labelminzoom) {
-        v_alpha = 0.0;
-    }
-    if (u_minfadezoom >= a_labelminzoom) {
-        v_alpha = 1.0;
-    }
-
-    // if label has been faded out, clip it
-    show *= (1.0 - step(v_alpha, 0.0));
-
     if (u_skewed) {
         vec4 extrude = u_exmatrix * vec4(a_offset * show / 64.0, 0, 0);
         gl_Position = u_matrix * vec4(a_pos + extrude.xy, 0, 1);
@@ -61,4 +39,5 @@ void main() {
     v_gamma_scale = (gl_Position.w - 0.5);
 
     v_tex = a_tex / u_texsize;
+    v_fade_tex = vec2(a_labelminzoom / 255.0, 0.0);
 }
diff --git a/src/mbgl/shader/sdf_shader.hpp b/src/mbgl/shader/sdf_shader.hpp
index 402168de0c..5fe6a44370 100644
--- a/src/mbgl/shader/sdf_shader.hpp
+++ b/src/mbgl/shader/sdf_shader.hpp
@@ -17,12 +17,9 @@ public:
     Uniform<GLfloat>                u_buffer      = {"u_buffer",      *this};
     Uniform<GLfloat>                u_gamma       = {"u_gamma",       *this};
     Uniform<GLfloat>                u_zoom        = {"u_zoom",        *this};
-    Uniform<GLfloat>                u_fadedist    = {"u_fadedist",    *this};
-    Uniform<GLfloat>                u_minfadezoom = {"u_minfadezoom", *this};
-    Uniform<GLfloat>                u_maxfadezoom = {"u_maxfadezoom", *this};
-    Uniform<GLfloat>                u_fadezoom    = {"u_fadezoom",    *this};
     Uniform<GLint>                  u_skewed      = {"u_skewed",      *this};
     Uniform<GLint>                  u_texture     = {"u_texture",     *this};
+    Uniform<GLint>                  u_fadetexture = {"u_fadetexture", *this};
 
 protected:
     GLint a_offset = -1;