[core] pack extrude and line distance into unsigned byte

instead of a signed byte. It's a bit easier to understand.

https://github.com/mapbox/mapbox-gl-js/commit/6c29251b3572ce9f2afae8182b56da6c1b9838b4

7 files changed, 24 insertions, 16 deletions

diff --git a/src/mbgl/geometry/line_buffer.cpp b/src/mbgl/geometry/line_buffer.cpp
index 75b9af7e20..3d979a2a45 100644
--- a/src/mbgl/geometry/line_buffer.cpp
+++ b/src/mbgl/geometry/line_buffer.cpp
@@ -13,16 +13,24 @@ GLsizei LineVertexBuffer::add(vertex_type x, vertex_type y, float ex, float ey,
     coords[0] = (x * 2) | tx;
     coords[1] = (y * 2) | ty;
 
-    int8_t *extrude = static_cast<int8_t *>(data);
-    extrude[4] = ::round(extrudeScale * ex);
-    extrude[5] = ::round(extrudeScale * ey);
+    uint8_t *ubytes = static_cast<uint8_t *>(data);
+    // add 128 to store an byte in an unsigned byte
+    ubytes[4] = ::round(extrudeScale * ex) + 128;
+    ubytes[5] = ::round(extrudeScale * ey) + 128;
+
+    // Encode the -1/0/1 direction value into the first two bits of .z of a_data.
+    // Combine it with the lower 6 bits of `linesofar` (shifted by 2 bites to make
+    // room for the direction value). The upper 8 bits of `linesofar` are placed in
+    // the `w` component. `linesofar` is scaled down by `LINE_DISTANCE_SCALE` so that
+    // we can store longer distances while sacrificing precision.
 
     // Encode the -1/0/1 direction value into .zw coordinates of a_data, which is normally covered
     // by linesofar, so we need to merge them.
     // The z component's first bit, as well as the sign bit is reserved for the direction,
     // so we need to shift the linesofar.
-    extrude[6] = ((dir < 0) ? -1 : 1) * ((dir ? 1 : 0) | static_cast<int8_t>((linesofar << 1) & 0x7F));
-    extrude[7] = ((linesofar >> 6) & 0xFF) - 128;
+
+    ubytes[6] = ((dir == 0 ? 0 : (dir < 0 ? -1 : 1 )) + 1) | ((linesofar & 0x3F) << 2);
+    ubytes[7] = linesofar >> 6;
 
     return idx;
 }
diff --git a/src/mbgl/shader/line.vertex.glsl b/src/mbgl/shader/line.vertex.glsl
index 1bd7f5ef5f..973e92a17d 100644
--- a/src/mbgl/shader/line.vertex.glsl
+++ b/src/mbgl/shader/line.vertex.glsl
@@ -23,8 +23,8 @@ varying vec2 v_normal;
 varying float v_gamma_scale;
 
 void main() {
-    vec2 a_extrude = a_data.xy;
-    float a_direction = sign(a_data.z) * mod(a_data.z, 2.0);
+    vec2 a_extrude = a_data.xy - 128.0;
+    float a_direction = mod(a_data.z, 4.0) - 1.0;
 
     // We store the texture normals in the most insignificant bit
     // transform y so that 0 => -1 and 1 => 1
diff --git a/src/mbgl/shader/line_shader.cpp b/src/mbgl/shader/line_shader.cpp
index e4df0531d8..c54b3312d8 100644
--- a/src/mbgl/shader/line_shader.cpp
+++ b/src/mbgl/shader/line_shader.cpp
@@ -17,5 +17,5 @@ void LineShader::bind(GLbyte* offset) {
     MBGL_CHECK_ERROR(glVertexAttribPointer(a_pos, 2, GL_SHORT, false, 8, offset + 0));
 
     MBGL_CHECK_ERROR(glEnableVertexAttribArray(a_data));
-    MBGL_CHECK_ERROR(glVertexAttribPointer(a_data, 4, GL_BYTE, false, 8, offset + 4));
+    MBGL_CHECK_ERROR(glVertexAttribPointer(a_data, 4, GL_UNSIGNED_BYTE, false, 8, offset + 4));
 }
diff --git a/src/mbgl/shader/linepattern.vertex.glsl b/src/mbgl/shader/linepattern.vertex.glsl
index 6fa499106c..5cceb64038 100644
--- a/src/mbgl/shader/linepattern.vertex.glsl
+++ b/src/mbgl/shader/linepattern.vertex.glsl
@@ -32,9 +32,9 @@ varying float v_linesofar;
 varying float v_gamma_scale;
 
 void main() {
-    vec2 a_extrude = a_data.xy;
-    float a_direction = sign(a_data.z) * mod(a_data.z, 2.0);
-    float a_linesofar = (abs(floor(a_data.z / 2.0)) + (a_data.w + 128.0) * 64.0) * LINE_DISTANCE_SCALE;
+    vec2 a_extrude = a_data.xy - 128.0;
+    float a_direction = mod(a_data.z, 4.0) - 1.0;
+    float a_linesofar = (floor(a_data.z / 4.0) + a_data.w * 64.0) * LINE_DISTANCE_SCALE;
 
     // We store the texture normals in the most insignificant bit
     // transform y so that 0 => -1 and 1 => 1
diff --git a/src/mbgl/shader/linepattern_shader.cpp b/src/mbgl/shader/linepattern_shader.cpp
index 7acd42f727..40f64ed139 100644
--- a/src/mbgl/shader/linepattern_shader.cpp
+++ b/src/mbgl/shader/linepattern_shader.cpp
@@ -17,5 +17,5 @@ void LinepatternShader::bind(GLbyte* offset) {
     MBGL_CHECK_ERROR(glVertexAttribPointer(a_pos, 2, GL_SHORT, false, 8, offset + 0));
 
     MBGL_CHECK_ERROR(glEnableVertexAttribArray(a_data));
-    MBGL_CHECK_ERROR(glVertexAttribPointer(a_data, 4, GL_BYTE, false, 8, offset + 4));
+    MBGL_CHECK_ERROR(glVertexAttribPointer(a_data, 4, GL_UNSIGNED_BYTE, false, 8, offset + 4));
 }
diff --git a/src/mbgl/shader/linesdf.vertex.glsl b/src/mbgl/shader/linesdf.vertex.glsl
index f24d8399b8..9174e4d987 100644
--- a/src/mbgl/shader/linesdf.vertex.glsl
+++ b/src/mbgl/shader/linesdf.vertex.glsl
@@ -36,9 +36,9 @@ varying vec2 v_tex_b;
 varying float v_gamma_scale;
 
 void main() {
-    vec2 a_extrude = a_data.xy;
-    float a_direction = sign(a_data.z) * mod(a_data.z, 2.0);
-    float a_linesofar = (abs(floor(a_data.z / 2.0)) + (a_data.w + 128.0) * 64.0) * LINE_DISTANCE_SCALE;
+    vec2 a_extrude = a_data.xy - 128.0;
+    float a_direction = mod(a_data.z, 4.0) - 1.0;
+    float a_linesofar = (floor(a_data.z / 4.0) + a_data.w * 64.0) * LINE_DISTANCE_SCALE;
 
     // We store the texture normals in the most insignificant bit
     // transform y so that 0 => -1 and 1 => 1
diff --git a/src/mbgl/shader/linesdf_shader.cpp b/src/mbgl/shader/linesdf_shader.cpp
index 00adaf0c86..1358127925 100644
--- a/src/mbgl/shader/linesdf_shader.cpp
+++ b/src/mbgl/shader/linesdf_shader.cpp
@@ -17,5 +17,5 @@ void LineSDFShader::bind(GLbyte* offset) {
     MBGL_CHECK_ERROR(glVertexAttribPointer(a_pos, 2, GL_SHORT, false, 8, offset + 0));
 
     MBGL_CHECK_ERROR(glEnableVertexAttribArray(a_data));
-    MBGL_CHECK_ERROR(glVertexAttribPointer(a_data, 4, GL_BYTE, false, 8, offset + 4));
+    MBGL_CHECK_ERROR(glVertexAttribPointer(a_data, 4, GL_UNSIGNED_BYTE, false, 8, offset + 4));
 }