[core] Always manually premultiply libpng output

It appears to be the only way to get the results we want in all cases.

2 files changed, 44 insertions, 0 deletions

diff --git a/src/mbgl/util/premultiply.cpp b/src/mbgl/util/premultiply.cpp
new file mode 100644
index 0000000000..3ee5a1e129
--- /dev/null
+++ b/src/mbgl/util/premultiply.cpp
@@ -0,0 +1,30 @@
+#include <mbgl/util/premultiply.hpp>
+
+#include <cmath>
+
+namespace mbgl {
+namespace util {
+
+PremultipliedImage premultiply(UnassociatedImage&& src) {
+   PremultipliedImage dst;
+
+   dst.width = src.width;
+   dst.height = src.height;
+   dst.data = std::move(src.data);
+
+   uint8_t* data = dst.data.get();
+   for (size_t i = 0; i < dst.size(); i += 4) {
+        uint8_t& r = data[i + 0];
+        uint8_t& g = data[i + 1];
+        uint8_t& b = data[i + 2];
+        uint8_t& a = data[i + 3];
+        r = (r * a + 127) / 255;
+        g = (g * a + 127) / 255;
+        b = (b * a + 127) / 255;
+    }
+
+    return std::move(dst);
+}
+
+}
+}
diff --git a/src/mbgl/util/premultiply.hpp b/src/mbgl/util/premultiply.hpp
new file mode 100644
index 0000000000..9d9dcc6468
--- /dev/null
+++ b/src/mbgl/util/premultiply.hpp
@@ -0,0 +1,14 @@
+#ifndef MBGL_UTIL_PREMULTIPLY
+#define MBGL_UTIL_PREMULTIPLY
+
+#include <mbgl/util/image.hpp>
+
+namespace mbgl {
+namespace util {
+
+PremultipliedImage premultiply(UnassociatedImage&&);
+
+}
+}
+
+#endif