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.

1 files changed, 25 insertions, 0 deletions

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)
 //