transition dashes when resetting their stretch

Dash patterns stretch from one integer zoom until the
next integer zoom where they reset. This transitions
those resets.

js: be5c7184bfd8cc9d47299041eb7fadbd41f6f6f0

src/mbgl/renderer/painter_line.cpp

@@ -92,10 +92,23 @@ void Painter::renderLine(LineBucket& bucket, util::ptr<StyleLayer> layer_desc, c
         float scaleX = patternratio / pos.width / properties.dash_line_width;
         float scaleY = -pos.height / 2.0;
 
-        linesdfShader->u_patternscale = {{ scaleX, scaleY }};
-        linesdfShader->u_tex_y = pos.y;
+        float mix;
+        if (state.getZoom() > lastIntegerZoom) {
+            // zooming in
+            mix = fraction + (1.0f - fraction) * t;
+            scaleX /= 2.0;
+        } else {
+            // zooming out
+            mix = fraction - fraction * t;
+        }
+
+        linesdfShader->u_patternscale_a = {{ scaleX, scaleY }};
+        linesdfShader->u_tex_y_a = pos.y;
+        linesdfShader->u_patternscale_b = {{ scaleX * 2.0f, scaleY }};
+        linesdfShader->u_tex_y_b = pos.y;
         linesdfShader->u_image = 0;
         linesdfShader->u_sdfgamma = lineAtlas.width / (properties.dash_line_width * pos.width * 256.0 * state.getPixelRatio());
+        linesdfShader->u_mix = mix;
 
         bucket.drawLineSDF(*linesdfShader);
 

src/mbgl/shader/linesdf.fragment.glsl

@@ -3,9 +3,11 @@ uniform vec4 u_color;
 uniform float u_blur;
 uniform sampler2D u_image;
 uniform float u_sdfgamma;
+uniform float u_mix;
 
 varying vec2 v_normal;
-varying vec2 v_tex;
+varying vec2 v_tex_a;
+varying vec2 v_tex_b;
 
 void main() {
     // Calculate the distance of the pixel from the line in pixels.
@@ -16,7 +18,9 @@ void main() {
     // (v_linewidth.s)
     float alpha = clamp(min(dist - (u_linewidth.t - u_blur), u_linewidth.s - dist) / u_blur, 0.0, 1.0);
 
-    float sdfdist = texture2D(u_image, v_tex).a;
+    float sdfdist_a = texture2D(u_image, v_tex_a).a;
+    float sdfdist_b = texture2D(u_image, v_tex_b).a;
+    float sdfdist = mix(sdfdist_a, sdfdist_b, u_mix);
     alpha *= smoothstep(0.5 - u_sdfgamma, 0.5 + u_sdfgamma, sdfdist);
 
     gl_FragColor = u_color * alpha;

src/mbgl/shader/linesdf.vertex.glsl

@@ -17,11 +17,14 @@ uniform mat4 u_exmatrix;
 // shared
 uniform float u_ratio;
 uniform vec2 u_linewidth;
-uniform vec2 u_patternscale;
-uniform float u_tex_y;
+uniform vec2 u_patternscale_a;
+uniform float u_tex_y_a;
+uniform vec2 u_patternscale_b;
+uniform float u_tex_y_b;
 
 varying vec2 v_normal;
-varying vec2 v_tex;
+varying vec2 v_tex_a;
+varying vec2 v_tex_b;
 
 void main() {
     vec2 a_extrude = a_data.xy;
@@ -45,5 +48,6 @@ void main() {
     // tile's zoom level.
     gl_Position = u_matrix * vec4(floor(a_pos * 0.5), 0.0, 1.0) + u_exmatrix * dist;
 
-    v_tex = vec2(a_linesofar * u_patternscale.x, normal.y * u_patternscale.y + u_tex_y);
+    v_tex_a = vec2(a_linesofar * u_patternscale_a.x, normal.y * u_patternscale_a.y + u_tex_y_a);
+    v_tex_b = vec2(a_linesofar * u_patternscale_b.x, normal.y * u_patternscale_b.y + u_tex_y_b);
 }

src/mbgl/shader/linesdf_shader.hpp

@@ -18,10 +18,13 @@ public:
     Uniform<std::array<float, 2>>  u_linewidth = {"u_linewidth", *this};
     Uniform<float>                 u_ratio     = {"u_ratio",     *this};
     Uniform<float>                 u_blur      = {"u_blur",      *this};
-    Uniform<std::array<float, 2>>  u_patternscale = { "u_patternscale", *this };
-    Uniform<float>                 u_tex_y     = {"u_tex_y",     *this};
+    Uniform<std::array<float, 2>>  u_patternscale_a = { "u_patternscale_a", *this };
+    Uniform<float>                 u_tex_y_a   = {"u_tex_y_a",     *this};
+    Uniform<std::array<float, 2>>  u_patternscale_b = { "u_patternscale_b", *this };
+    Uniform<float>                 u_tex_y_b   = {"u_tex_y_b",     *this};
     Uniform<int32_t>               u_image     = {"u_image",     *this};
     Uniform<float>                 u_sdfgamma  = {"u_sdfgamma",  *this};
+    Uniform<float>                 u_mix       = {"u_mix",       *this};
 
 private:
     int32_t a_pos = -1;