add canvas:sub method

src/graphics/core/ImageData.cpp

@@ -448,7 +448,7 @@ void ImageData::mulColor(const glm::ivec4& color) {
     }
 }
 
-void ImageData::addColor(const ImageData& other) {
+void ImageData::addColor(const ImageData& other, int multiplier) {
     check_matching(*this, other);
 
     uint comps;
@@ -462,7 +462,7 @@ void ImageData::addColor(const ImageData& other) {
         for (uint x = 0; x < width; x++) {
             uint idx = (y * width + x) * comps;
             for (uint c = 0; c < comps; c++) {
-                int val = data[idx + c] + other.data[idx + c];
+                int val = data[idx + c] + other.data[idx + c] * multiplier;
                 data[idx + c] =
                     static_cast<ubyte>(std::min(std::max(val, 0), 255));
             }
@@ -470,7 +470,7 @@ void ImageData::addColor(const ImageData& other) {
     }
 }
 
-void ImageData::addColor(const glm::ivec4& color) {
+void ImageData::addColor(const glm::ivec4& color, int multiplier) {
     uint comps;
     switch (format) {
         case ImageFormat::rgb888: comps = 3; break;
@@ -482,7 +482,7 @@ void ImageData::addColor(const glm::ivec4& color) {
         for (uint x = 0; x < width; x++) {
             uint idx = (y * width + x) * comps;
             for (uint c = 0; c < comps; c++) {
-                int val = data[idx + c] + color[c];
+                int val = data[idx + c] + color[c] * multiplier;
                 data[idx + c] =
                     static_cast<ubyte>(std::min(std::max(val, 0), 255));
             }

src/graphics/core/ImageData.hpp

@@ -34,8 +34,8 @@ public:
     void fixAlphaColor();
     void mulColor(const glm::ivec4& color);
     void mulColor(const ImageData& other);
-    void addColor(const glm::ivec4& color);
-    void addColor(const ImageData& other);
+    void addColor(const glm::ivec4& color, int multiplier);
+    void addColor(const ImageData& other, int multiplier);
 
     std::unique_ptr<ImageData> cropped(int x, int y, int width, int height) const;
 

src/logic/scripting/lua/usertypes/lua_type_canvas.cpp

@@ -261,9 +261,23 @@ static int l_add(State* L) {
     }
     if (lua::istable(L, 2)) {
         RGBA rgba = get_rgba(L, 2);
-        canvas->getData().addColor(glm::ivec4 {rgba.r, rgba.g, rgba.b, rgba.a});
+        canvas->getData().addColor(glm::ivec4 {rgba.r, rgba.g, rgba.b, rgba.a}, 1);
     } else if (auto other = touserdata<LuaCanvas>(L, 2)) {
-        canvas->getData().addColor(other->getData());
+        canvas->getData().addColor(other->getData(), 1);
+    }
+    return 0;
+}
+
+static int l_sub(State* L) {
+    auto canvas = touserdata<LuaCanvas>(L, 1);
+    if (canvas == nullptr) {
+        return 0;
+    }
+    if (lua::istable(L, 2)) {
+        RGBA rgba = get_rgba(L, 2);
+        canvas->getData().addColor(glm::ivec4 {rgba.r, rgba.g, rgba.b, rgba.a}, -1);
+    } else if (auto other = touserdata<LuaCanvas>(L, 2)) {
+        canvas->getData().addColor(other->getData(), -1);
     }
     return 0;
 }
@@ -279,6 +293,7 @@ static std::unordered_map<std::string, lua_CFunction> methods {
     {"unbind_texture", lua::wrap<l_unbind_texture>},
     {"mul", lua::wrap<l_mul>},
     {"add", lua::wrap<l_add>},
+    {"sub", lua::wrap<l_sub>},
     {"_set_data", lua::wrap<l_set_data>},
 };