[core] Simplify and fix sprite atlas coordinate calculations

* Always return image metrics exclusive of padding
* Work with integer coordinates whenever possible
* Eliminate redundant SpriteAtlasElement members
* Fix asymmetric re-padding in getIconQuad when pixelRatio != 1
* Add explanatory comments

2 files changed, 21 insertions, 9 deletions

diff --git a/src/mbgl/text/quads.cpp b/src/mbgl/text/quads.cpp
index dc48634fb0..33b94d7114 100644
--- a/src/mbgl/text/quads.cpp
+++ b/src/mbgl/text/quads.cpp
@@ -24,11 +24,15 @@ SymbolQuad getIconQuad(const Anchor& anchor,
                        const Shaping& shapedText) {
     const SpriteAtlasElement& image = shapedIcon.image();
 
+    // If you have a 10px icon that isn't perfectly aligned to the pixel grid it will cover 11 actual
+    // pixels. The quad needs to be padded to account for this, otherwise they'll look slightly clipped
+    // on one edge in some cases.
     const float border = 1.0;
-    auto left = shapedIcon.left() - border;
-    auto right = left + image.pos.w / image.relativePixelRatio;
-    auto top = shapedIcon.top() - border;
-    auto bottom = top + image.pos.h / image.relativePixelRatio;
+
+    float top = shapedIcon.top() - border / image.pixelRatio;
+    float left = shapedIcon.left() - border / image.pixelRatio;
+    float bottom = shapedIcon.bottom() + border / image.pixelRatio;
+    float right = shapedIcon.right() + border / image.pixelRatio;
     Point<float> tl;
     Point<float> tr;
     Point<float> br;
@@ -92,7 +96,15 @@ SymbolQuad getIconQuad(const Anchor& anchor,
         br = util::matrixMultiply(matrix, br);
     }
 
-    return SymbolQuad { tl, tr, bl, br, image.pos, 0, 0, anchor.point, globalMinScale, std::numeric_limits<float>::infinity(), shapedText.writingMode };
+    // Icon quad is padded, so texture coordinates also need to be padded.
+    Rect<uint16_t> textureRect {
+        static_cast<uint16_t>(image.textureRect.x - border),
+        static_cast<uint16_t>(image.textureRect.y - border),
+        static_cast<uint16_t>(image.textureRect.w + border * 2),
+        static_cast<uint16_t>(image.textureRect.h + border * 2)
+    };
+
+    return SymbolQuad { tl, tr, bl, br, textureRect, 0, 0, anchor.point, globalMinScale, std::numeric_limits<float>::infinity(), shapedText.writingMode };
 }
 
 struct GlyphInstance {
diff --git a/src/mbgl/text/shaping.cpp b/src/mbgl/text/shaping.cpp
index 6ee646be96..f9f90627d4 100644
--- a/src/mbgl/text/shaping.cpp
+++ b/src/mbgl/text/shaping.cpp
@@ -13,10 +13,10 @@ namespace mbgl {
 PositionedIcon PositionedIcon::shapeIcon(const SpriteAtlasElement& image, const std::array<float, 2>& iconOffset, const float iconRotation) {
     float dx = iconOffset[0];
     float dy = iconOffset[1];
-    float x1 = dx - image.size[0] / 2.0f;
-    float x2 = x1 + image.size[0];
-    float y1 = dy - image.size[1] / 2.0f;
-    float y2 = y1 + image.size[1];
+    float x1 = dx - image.displaySize()[0] / 2.0f;
+    float x2 = x1 + image.displaySize()[0];
+    float y1 = dy - image.displaySize()[1] / 2.0f;
+    float y2 = y1 + image.displaySize()[1];
 
     return PositionedIcon { image, y1, y2, x1, x2, iconRotation };
 }