Pack min + max into one attribute :muscle:

Some devices supported by Mapbox GL provide only 8 vertex attributes; this change packs existing attributes to get us just under that limit.

For properties using a composite function, pack the min and max values into a single attribute with two logical components instead of using two separate attributes and buffers. Special logic is included for color attributes, whose integer components must be packed into the available bits of floating-point attributes. (We don't have access to ivec types in GL ES 2.0.)

For source functions, continue to bind just a one-component attribute even though the GLSL type is vec2 (or vec4 for colors). The type-checking done by gl::Attribute is relaxed slightly to accommodate this.

11 files changed, 100 insertions, 103 deletions

diff --git a/src/mbgl/shaders/circle.cpp b/src/mbgl/shaders/circle.cpp
index 4b2bb7cecf..b479f1d40c 100644
--- a/src/mbgl/shaders/circle.cpp
+++ b/src/mbgl/shaders/circle.cpp
@@ -14,45 +14,38 @@ uniform vec2 u_extrude_scale;
 attribute vec2 a_pos;
 
 uniform lowp float a_color_t;
-attribute lowp vec4 a_color_min;
-attribute lowp vec4 a_color_max;
+attribute lowp vec4 a_color;
 varying lowp vec4 color;
 uniform lowp float a_radius_t;
-attribute mediump float a_radius_min;
-attribute mediump float a_radius_max;
+attribute mediump vec2 a_radius;
 varying mediump float radius;
 uniform lowp float a_blur_t;
-attribute lowp float a_blur_min;
-attribute lowp float a_blur_max;
+attribute lowp vec2 a_blur;
 varying lowp float blur;
 uniform lowp float a_opacity_t;
-attribute lowp float a_opacity_min;
-attribute lowp float a_opacity_max;
+attribute lowp vec2 a_opacity;
 varying lowp float opacity;
 uniform lowp float a_stroke_color_t;
-attribute lowp vec4 a_stroke_color_min;
-attribute lowp vec4 a_stroke_color_max;
+attribute lowp vec4 a_stroke_color;
 varying lowp vec4 stroke_color;
 uniform lowp float a_stroke_width_t;
-attribute mediump float a_stroke_width_min;
-attribute mediump float a_stroke_width_max;
+attribute mediump vec2 a_stroke_width;
 varying mediump float stroke_width;
 uniform lowp float a_stroke_opacity_t;
-attribute lowp float a_stroke_opacity_min;
-attribute lowp float a_stroke_opacity_max;
+attribute lowp vec2 a_stroke_opacity;
 varying lowp float stroke_opacity;
 
 varying vec2 v_extrude;
 varying lowp float v_antialiasblur;
 
 void main(void) {
-    color = mix(a_color_min, a_color_max, a_color_t);
-    radius = mix(a_radius_min, a_radius_max, a_radius_t);
-    blur = mix(a_blur_min, a_blur_max, a_blur_t);
-    opacity = mix(a_opacity_min, a_opacity_max, a_opacity_t);
-    stroke_color = mix(a_stroke_color_min, a_stroke_color_max, a_stroke_color_t);
-    stroke_width = mix(a_stroke_width_min, a_stroke_width_max, a_stroke_width_t);
-    stroke_opacity = mix(a_stroke_opacity_min, a_stroke_opacity_max, a_stroke_opacity_t);
+    color = unpack_mix_vec4(a_color, a_color_t);
+    radius = unpack_mix_vec2(a_radius, a_radius_t);
+    blur = unpack_mix_vec2(a_blur, a_blur_t);
+    opacity = unpack_mix_vec2(a_opacity, a_opacity_t);
+    stroke_color = unpack_mix_vec4(a_stroke_color, a_stroke_color_t);
+    stroke_width = unpack_mix_vec2(a_stroke_width, a_stroke_width_t);
+    stroke_opacity = unpack_mix_vec2(a_stroke_opacity, a_stroke_opacity_t);
 
     // unencode the extrusion vector that we snuck into the a_pos vector
     v_extrude = vec2(mod(a_pos, 2.0) * 2.0 - 1.0);
diff --git a/src/mbgl/shaders/fill.cpp b/src/mbgl/shaders/fill.cpp
index d0fc3a7033..a1fba4d749 100644
--- a/src/mbgl/shaders/fill.cpp
+++ b/src/mbgl/shaders/fill.cpp
@@ -12,17 +12,15 @@ attribute vec2 a_pos;
 uniform mat4 u_matrix;
 
 uniform lowp float a_color_t;
-attribute lowp vec4 a_color_min;
-attribute lowp vec4 a_color_max;
+attribute lowp vec4 a_color;
 varying lowp vec4 color;
 uniform lowp float a_opacity_t;
-attribute lowp float a_opacity_min;
-attribute lowp float a_opacity_max;
+attribute lowp vec2 a_opacity;
 varying lowp float opacity;
 
 void main() {
-    color = mix(a_color_min, a_color_max, a_color_t);
-    opacity = mix(a_opacity_min, a_opacity_max, a_opacity_t);
+    color = unpack_mix_vec4(a_color, a_color_t);
+    opacity = unpack_mix_vec2(a_opacity, a_opacity_t);
 
     gl_Position = u_matrix * vec4(a_pos, 0, 1);
 }
diff --git a/src/mbgl/shaders/fill_outline.cpp b/src/mbgl/shaders/fill_outline.cpp
index d6f8d63715..74201b518d 100644
--- a/src/mbgl/shaders/fill_outline.cpp
+++ b/src/mbgl/shaders/fill_outline.cpp
@@ -15,17 +15,15 @@ uniform vec2 u_world;
 varying vec2 v_pos;
 
 uniform lowp float a_outline_color_t;
-attribute lowp vec4 a_outline_color_min;
-attribute lowp vec4 a_outline_color_max;
+attribute lowp vec4 a_outline_color;
 varying lowp vec4 outline_color;
 uniform lowp float a_opacity_t;
-attribute lowp float a_opacity_min;
-attribute lowp float a_opacity_max;
+attribute lowp vec2 a_opacity;
 varying lowp float opacity;
 
 void main() {
-    outline_color = mix(a_outline_color_min, a_outline_color_max, a_outline_color_t);
-    opacity = mix(a_opacity_min, a_opacity_max, a_opacity_t);
+    outline_color = unpack_mix_vec4(a_outline_color, a_outline_color_t);
+    opacity = unpack_mix_vec2(a_opacity, a_opacity_t);
 
     gl_Position = u_matrix * vec4(a_pos, 0, 1);
     v_pos = (gl_Position.xy / gl_Position.w + 1.0) / 2.0 * u_world;
diff --git a/src/mbgl/shaders/fill_outline_pattern.cpp b/src/mbgl/shaders/fill_outline_pattern.cpp
index 6e9d6ad9a1..5e38023382 100644
--- a/src/mbgl/shaders/fill_outline_pattern.cpp
+++ b/src/mbgl/shaders/fill_outline_pattern.cpp
@@ -24,12 +24,11 @@ varying vec2 v_pos_b;
 varying vec2 v_pos;
 
 uniform lowp float a_opacity_t;
-attribute lowp float a_opacity_min;
-attribute lowp float a_opacity_max;
+attribute lowp vec2 a_opacity;
 varying lowp float opacity;
 
 void main() {
-    opacity = mix(a_opacity_min, a_opacity_max, a_opacity_t);
+    opacity = unpack_mix_vec2(a_opacity, a_opacity_t);
 
     gl_Position = u_matrix * vec4(a_pos, 0, 1);
 
diff --git a/src/mbgl/shaders/fill_pattern.cpp b/src/mbgl/shaders/fill_pattern.cpp
index 329a292eed..0357fed40e 100644
--- a/src/mbgl/shaders/fill_pattern.cpp
+++ b/src/mbgl/shaders/fill_pattern.cpp
@@ -22,12 +22,11 @@ varying vec2 v_pos_a;
 varying vec2 v_pos_b;
 
 uniform lowp float a_opacity_t;
-attribute lowp float a_opacity_min;
-attribute lowp float a_opacity_max;
+attribute lowp vec2 a_opacity;
 varying lowp float opacity;
 
 void main() {
-    opacity = mix(a_opacity_min, a_opacity_max, a_opacity_t);
+    opacity = unpack_mix_vec2(a_opacity, a_opacity_t);
 
     gl_Position = u_matrix * vec4(a_pos, 0, 1);
 
diff --git a/src/mbgl/shaders/line.cpp b/src/mbgl/shaders/line.cpp
index bd3256ad3b..dc4aa774dc 100644
--- a/src/mbgl/shaders/line.cpp
+++ b/src/mbgl/shaders/line.cpp
@@ -34,32 +34,27 @@ varying vec2 v_width2;
 varying float v_gamma_scale;
 
 uniform lowp float a_color_t;
-attribute lowp vec4 a_color_min;
-attribute lowp vec4 a_color_max;
+attribute lowp vec4 a_color;
 varying lowp vec4 color;
 uniform lowp float a_blur_t;
-attribute lowp float a_blur_min;
-attribute lowp float a_blur_max;
+attribute lowp vec2 a_blur;
 varying lowp float blur;
 uniform lowp float a_opacity_t;
-attribute lowp float a_opacity_min;
-attribute lowp float a_opacity_max;
+attribute lowp vec2 a_opacity;
 varying lowp float opacity;
 uniform lowp float a_gapwidth_t;
-attribute mediump float a_gapwidth_min;
-attribute mediump float a_gapwidth_max;
+attribute mediump vec2 a_gapwidth;
 varying mediump float gapwidth;
 uniform lowp float a_offset_t;
-attribute lowp float a_offset_min;
-attribute lowp float a_offset_max;
+attribute lowp vec2 a_offset;
 varying lowp float offset;
 
 void main() {
-    color = mix(a_color_min, a_color_max, a_color_t);
-    blur = mix(a_blur_min, a_blur_max, a_blur_t);
-    opacity = mix(a_opacity_min, a_opacity_max, a_opacity_t);
-    gapwidth = mix(a_gapwidth_min, a_gapwidth_max, a_gapwidth_t);
-    offset = mix(a_offset_min, a_offset_max, a_offset_t);
+    color = unpack_mix_vec4(a_color, a_color_t);
+    blur = unpack_mix_vec2(a_blur, a_blur_t);
+    opacity = unpack_mix_vec2(a_opacity, a_opacity_t);
+    gapwidth = unpack_mix_vec2(a_gapwidth, a_gapwidth_t);
+    offset = unpack_mix_vec2(a_offset, a_offset_t);
 
     vec2 a_extrude = a_data.xy - 128.0;
     float a_direction = mod(a_data.z, 4.0) - 1.0;
diff --git a/src/mbgl/shaders/line_pattern.cpp b/src/mbgl/shaders/line_pattern.cpp
index 2be98deab3..f52a8e2157 100644
--- a/src/mbgl/shaders/line_pattern.cpp
+++ b/src/mbgl/shaders/line_pattern.cpp
@@ -37,27 +37,23 @@ varying float v_linesofar;
 varying float v_gamma_scale;
 
 uniform lowp float a_blur_t;
-attribute lowp float a_blur_min;
-attribute lowp float a_blur_max;
+attribute lowp vec2 a_blur;
 varying lowp float blur;
 uniform lowp float a_opacity_t;
-attribute lowp float a_opacity_min;
-attribute lowp float a_opacity_max;
+attribute lowp vec2 a_opacity;
 varying lowp float opacity;
 uniform lowp float a_offset_t;
-attribute lowp float a_offset_min;
-attribute lowp float a_offset_max;
+attribute lowp vec2 a_offset;
 varying lowp float offset;
 uniform lowp float a_gapwidth_t;
-attribute mediump float a_gapwidth_min;
-attribute mediump float a_gapwidth_max;
+attribute mediump vec2 a_gapwidth;
 varying mediump float gapwidth;
 
 void main() {
-    blur = mix(a_blur_min, a_blur_max, a_blur_t);
-    opacity = mix(a_opacity_min, a_opacity_max, a_opacity_t);
-    offset = mix(a_offset_min, a_offset_max, a_offset_t);
-    gapwidth = mix(a_gapwidth_min, a_gapwidth_max, a_gapwidth_t);
+    blur = unpack_mix_vec2(a_blur, a_blur_t);
+    opacity = unpack_mix_vec2(a_opacity, a_opacity_t);
+    offset = unpack_mix_vec2(a_offset, a_offset_t);
+    gapwidth = unpack_mix_vec2(a_gapwidth, a_gapwidth_t);
 
     vec2 a_extrude = a_data.xy - 128.0;
     float a_direction = mod(a_data.z, 4.0) - 1.0;
diff --git a/src/mbgl/shaders/line_sdf.cpp b/src/mbgl/shaders/line_sdf.cpp
index 5f01cf00c0..cd0d4ac318 100644
--- a/src/mbgl/shaders/line_sdf.cpp
+++ b/src/mbgl/shaders/line_sdf.cpp
@@ -42,32 +42,27 @@ varying vec2 v_tex_b;
 varying float v_gamma_scale;
 
 uniform lowp float a_color_t;
-attribute lowp vec4 a_color_min;
-attribute lowp vec4 a_color_max;
+attribute lowp vec4 a_color;
 varying lowp vec4 color;
 uniform lowp float a_blur_t;
-attribute lowp float a_blur_min;
-attribute lowp float a_blur_max;
+attribute lowp vec2 a_blur;
 varying lowp float blur;
 uniform lowp float a_opacity_t;
-attribute lowp float a_opacity_min;
-attribute lowp float a_opacity_max;
+attribute lowp vec2 a_opacity;
 varying lowp float opacity;
 uniform lowp float a_gapwidth_t;
-attribute mediump float a_gapwidth_min;
-attribute mediump float a_gapwidth_max;
+attribute mediump vec2 a_gapwidth;
 varying mediump float gapwidth;
 uniform lowp float a_offset_t;
-attribute lowp float a_offset_min;
-attribute lowp float a_offset_max;
+attribute lowp vec2 a_offset;
 varying lowp float offset;
 
 void main() {
-    color = mix(a_color_min, a_color_max, a_color_t);
-    blur = mix(a_blur_min, a_blur_max, a_blur_t);
-    opacity = mix(a_opacity_min, a_opacity_max, a_opacity_t);
-    gapwidth = mix(a_gapwidth_min, a_gapwidth_max, a_gapwidth_t);
-    offset = mix(a_offset_min, a_offset_max, a_offset_t);
+    color = unpack_mix_vec4(a_color, a_color_t);
+    blur = unpack_mix_vec2(a_blur, a_blur_t);
+    opacity = unpack_mix_vec2(a_opacity, a_opacity_t);
+    gapwidth = unpack_mix_vec2(a_gapwidth, a_gapwidth_t);
+    offset = unpack_mix_vec2(a_offset, a_offset_t);
 
     vec2 a_extrude = a_data.xy - 128.0;
     float a_direction = mod(a_data.z, 4.0) - 1.0;
diff --git a/src/mbgl/shaders/preludes.cpp b/src/mbgl/shaders/preludes.cpp
index a0e9414ceb..806e655285 100644
--- a/src/mbgl/shaders/preludes.cpp
+++ b/src/mbgl/shaders/preludes.cpp
@@ -43,6 +43,31 @@ vec4 evaluate_zoom_function_4(const vec4 value0, const vec4 value1, const vec4 v
     }
 }
 
+
+// To minimize the number of attributes needed in the mapbox-gl-native shaders,
+// we encode a 4-component color into a pair of floats (i.e. a vec2) as follows:
+// [ floor(color.r * 255) * 256 + color.g * 255,
+//   floor(color.b * 255) * 256 + color.g * 255 ]
+vec4 decode_color(const vec2 encodedColor) {
+    float r = floor(encodedColor[0]/256.0)/255.0;
+    float g = (encodedColor[0] - r*256.0*255.0)/255.0;
+    float b = floor(encodedColor[1]/256.0)/255.0;
+    float a = (encodedColor[1] - b*256.0*255.0)/255.0;
+    return vec4(r, g, b, a);
+}
+
+// Unpack a pair of paint values and interpolate between them.
+float unpack_mix_vec2(const vec2 packedValue, const float t) {
+    return mix(packedValue[0], packedValue[1], t);
+}
+
+// Unpack a pair of paint values and interpolate between them.
+vec4 unpack_mix_vec4(const vec4 packedColors, const float t) {
+    vec4 minColor = decode_color(vec2(packedColors[0], packedColors[1]));
+    vec4 maxColor = decode_color(vec2(packedColors[2], packedColors[3]));
+    return mix(minColor, maxColor, t);
+}
+
 // The offset depends on how many pixels are between the world origin and the edge of the tile:
 // vec2 offset = mod(pixel_coord, size)
 //
diff --git a/src/mbgl/shaders/symbol_icon.cpp b/src/mbgl/shaders/symbol_icon.cpp
index 5945f1f3b1..9adda0ba16 100644
--- a/src/mbgl/shaders/symbol_icon.cpp
+++ b/src/mbgl/shaders/symbol_icon.cpp
@@ -7,14 +7,13 @@ namespace shaders {
 
 const char* symbol_icon::name = "symbol_icon";
 const char* symbol_icon::vertexSource = R"MBGL_SHADER(
-attribute vec2 a_pos;
-attribute vec2 a_offset;
+
+attribute vec4 a_pos_offset;
 attribute vec2 a_texture_pos;
 attribute vec4 a_data;
 
 uniform lowp float a_opacity_t;
-attribute lowp float a_opacity_min;
-attribute lowp float a_opacity_max;
+attribute lowp vec2 a_opacity;
 varying lowp float opacity;
 
 // matrix is for the vertex position.
@@ -30,7 +29,10 @@ varying vec2 v_tex;
 varying vec2 v_fade_tex;
 
 void main() {
-    opacity = mix(a_opacity_min, a_opacity_max, a_opacity_t);
+    opacity = unpack_mix_vec2(a_opacity, a_opacity_t);
+
+    vec2 a_pos = a_pos_offset.xy;
+    vec2 a_offset = a_pos_offset.zw;
 
     vec2 a_tex = a_texture_pos.xy;
     mediump float a_labelminzoom = a_data[0];
diff --git a/src/mbgl/shaders/symbol_sdf.cpp b/src/mbgl/shaders/symbol_sdf.cpp
index ccb4b9cee6..76399dbb55 100644
--- a/src/mbgl/shaders/symbol_sdf.cpp
+++ b/src/mbgl/shaders/symbol_sdf.cpp
@@ -9,30 +9,24 @@ const char* symbol_sdf::name = "symbol_sdf";
 const char* symbol_sdf::vertexSource = R"MBGL_SHADER(
 const float PI = 3.141592653589793;
 
-attribute vec2 a_pos;
-attribute vec2 a_offset;
+attribute vec4 a_pos_offset;
 attribute vec2 a_texture_pos;
 attribute vec4 a_data;
 
 uniform lowp float a_fill_color_t;
-attribute lowp vec4 a_fill_color_min;
-attribute lowp vec4 a_fill_color_max;
+attribute lowp vec4 a_fill_color;
 varying lowp vec4 fill_color;
 uniform lowp float a_halo_color_t;
-attribute lowp vec4 a_halo_color_min;
-attribute lowp vec4 a_halo_color_max;
+attribute lowp vec4 a_halo_color;
 varying lowp vec4 halo_color;
 uniform lowp float a_opacity_t;
-attribute lowp float a_opacity_min;
-attribute lowp float a_opacity_max;
+attribute lowp vec2 a_opacity;
 varying lowp float opacity;
 uniform lowp float a_halo_width_t;
-attribute lowp float a_halo_width_min;
-attribute lowp float a_halo_width_max;
+attribute lowp vec2 a_halo_width;
 varying lowp float halo_width;
 uniform lowp float a_halo_blur_t;
-attribute lowp float a_halo_blur_min;
-attribute lowp float a_halo_blur_max;
+attribute lowp vec2 a_halo_blur;
 varying lowp float halo_blur;
 
 // matrix is for the vertex position.
@@ -53,11 +47,14 @@ varying vec2 v_fade_tex;
 varying float v_gamma_scale;
 
 void main() {
-    fill_color = mix(a_fill_color_min, a_fill_color_max, a_fill_color_t);
-    halo_color = mix(a_halo_color_min, a_halo_color_max, a_halo_color_t);
-    opacity = mix(a_opacity_min, a_opacity_max, a_opacity_t);
-    halo_width = mix(a_halo_width_min, a_halo_width_max, a_halo_width_t);
-    halo_blur = mix(a_halo_blur_min, a_halo_blur_max, a_halo_blur_t);
+    fill_color = unpack_mix_vec4(a_fill_color, a_fill_color_t);
+    halo_color = unpack_mix_vec4(a_halo_color, a_halo_color_t);
+    opacity = unpack_mix_vec2(a_opacity, a_opacity_t);
+    halo_width = unpack_mix_vec2(a_halo_width, a_halo_width_t);
+    halo_blur = unpack_mix_vec2(a_halo_blur, a_halo_blur_t);
+
+    vec2 a_pos = a_pos_offset.xy;
+    vec2 a_offset = a_pos_offset.zw;
 
     vec2 a_tex = a_texture_pos.xy;
     mediump float a_labelminzoom = a_data[0];