Found the problem with openmp.

X here need to be declared as private, else each thread are sharing the same variable which... well.... don't work well .. :/

source/camera.cpp

@@ -62,9 +62,11 @@ Canvas Camera::render(World world, uint32_t depth)
     uint32_t x, y;
     Canvas image = Canvas(this->horizontalSize, this->verticalSize);
 
+#pragma omp parallel private(x, y) shared(image)
+    {
+#pragma omp for
         for (y = 0 ; y < this->verticalSize ; y++)
         {
-        #pragma omp parallel for
             for (x = 0 ; x < this->horizontalSize ; x++)
             {
                 Ray r = this->rayForPixel(x, y);
@@ -72,6 +74,7 @@ Canvas Camera::render(World world, uint32_t depth)
                 image.putPixel(x, y, colour);
             }
         }
+    }
 
     return image;
 }