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

@@ -62,16 +62,19 @@ Canvas Camera::render(World world, uint32_t depth)
     uint32_t x, y;
     Canvas image = Canvas(this->horizontalSize, this->verticalSize);
 
-    for(y = 0; y < this->verticalSize; y++)
+#pragma omp parallel private(x, y) shared(image)
     {
-        #pragma omp parallel for
-        for(x = 0; x < this->horizontalSize; x++)
+#pragma omp for
+        for (y = 0 ; y < this->verticalSize ; y++)
+        {
+            for (x = 0 ; x < this->horizontalSize ; x++)
             {
                 Ray r = this->rayForPixel(x, y);
                 Tuple colour = world.colourAt(r, depth);
                 image.putPixel(x, y, colour);
             }
         }
+    }
 
     return image;
 }