[core] Fix image type of Map::renderStill

It's a premultiplied image. This implies that we were misusing encodePNG
in most cases, as we were passing premultiplied pixels which were then
interpreted as unmultiplied. I changed encodePNG to accept premultipled
pixels, and unpremultiply in the implementations.

3 files changed, 31 insertions, 7 deletions

diff --git a/src/mbgl/map/view.cpp b/src/mbgl/map/view.cpp
index fa8b7584cc..efbe0f1672 100644
--- a/src/mbgl/map/view.cpp
+++ b/src/mbgl/map/view.cpp
@@ -10,7 +10,7 @@ void View::initialize(Map *map_) {
     map = map_;
 }
 
-UnassociatedImage View::readStillImage() {
+PremultipliedImage View::readStillImage() {
     return {};
 }
 
diff --git a/src/mbgl/util/premultiply.cpp b/src/mbgl/util/premultiply.cpp
index 3ee5a1e129..8a62012251 100644
--- a/src/mbgl/util/premultiply.cpp
+++ b/src/mbgl/util/premultiply.cpp
@@ -6,14 +6,14 @@ namespace mbgl {
 namespace util {
 
 PremultipliedImage premultiply(UnassociatedImage&& src) {
-   PremultipliedImage dst;
+    PremultipliedImage dst;
 
-   dst.width = src.width;
-   dst.height = src.height;
-   dst.data = std::move(src.data);
+    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* 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];
@@ -26,5 +26,28 @@ PremultipliedImage premultiply(UnassociatedImage&& src) {
     return std::move(dst);
 }
 
+UnassociatedImage unpremultiply(PremultipliedImage&& src) {
+    UnassociatedImage 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];
+        if (a) {
+            r = (255 * r + (a / 2)) / a;
+            g = (255 * g + (a / 2)) / a;
+            b = (255 * b + (a / 2)) / a;
+        }
+    }
+
+    return std::move(dst);
+}
+
 }
 }
diff --git a/src/mbgl/util/premultiply.hpp b/src/mbgl/util/premultiply.hpp
index 9d9dcc6468..8d87c157c8 100644
--- a/src/mbgl/util/premultiply.hpp
+++ b/src/mbgl/util/premultiply.hpp
@@ -7,6 +7,7 @@ namespace mbgl {
 namespace util {
 
 PremultipliedImage premultiply(UnassociatedImage&&);
+UnassociatedImage unpremultiply(PremultipliedImage&&);
 
 }
 }