Incorporate changes to chooseSwapExtent

maltekliemann · maltekliemann · commit 91f969b4de08 · 2020-10-11T10:13:49.000+02:00
diff --git a/en/03_Drawing_a_triangle/01_Presentation/01_Swap_chain.md b/en/03_Drawing_a_triangle/01_Presentation/01_Swap_chain.md
@@ -318,14 +318,28 @@ VkExtent2D chooseSwapExtent(const VkSurfaceCapabilitiesKHR& capabilities) {
 ```
 
 The swap extent is the resolution of the swap chain images and it's almost
-always exactly equal to the resolution of the window that we're drawing to. The
-range of the possible resolutions is defined in the `VkSurfaceCapabilitiesKHR`
-structure. Vulkan tells us to match the resolution of the window by setting the
-width and height in the `currentExtent` member. However, some window managers do
-allow us to differ here and this is indicated by setting the width and height in
-`currentExtent` to a special value: the maximum value of `uint32_t`. In that
-case we'll pick the resolution that best matches the window within the
-`minImageExtent` and `maxImageExtent` bounds.
+always exactly equal to the resolution of the window that we're drawing to _in
+pixels_ (more on that in a moment). The range of the possible resolutions is
+defined in the `VkSurfaceCapabilitiesKHR` structure. Vulkan tells us to match
+the resolution of the window by setting the width and height in the
+`currentExtent` member. However, some window managers do allow us to differ here
+and this is indicated by setting the width and height in `currentExtent` to a
+special value: the maximum value of `uint32_t`. In that case we'll pick the
+resolution that best matches the window within the `minImageExtent` and
+`maxImageExtent` bounds. But we must specify the resolution in the correct unit.
+
+GLFW uses two units when measuring sizes: pixels and
+[screen coordinates](https://www.glfw.org/docs/latest/intro_guide.html#coordinate_systems).
+For example, the resolution `{WIDTH, HEIGHT}` that we specified earlier when
+creating the window is measured in screen coordinates. But Vulkan works with
+pixels, so the swap chain extent must be specified in pixels as well.
+Unfortunately, if you are using a high DPI display (like Apple's Retina
+display), screen coordinates don't correspond to pixels. Instead, due to the
+higher pixel density, the resolution of the window in pixel will be larger than
+the resolution in screen coordinates. So if Vulkan doesn't fix the swap extent
+for us, we can't just use the original `{WIDTH, HEIGHT}`. Instead, we must use
+`glfwGetFramebufferSize` to query the resolution of the window in pixel before
+matching it against the minimum and maximum image extent.
 
 ```c++
 #include <cstdint> // Necessary for UINT32_MAX
@@ -336,7 +350,13 @@ VkExtent2D chooseSwapExtent(const VkSurfaceCapabilitiesKHR& capabilities) {
     if (capabilities.currentExtent.width != UINT32_MAX) {
         return capabilities.currentExtent;
     } else {
-        VkExtent2D actualExtent = {WIDTH, HEIGHT};
+        int width, height;
+        glfwGetFramebufferSize(window, &width, &height);
+
+        VkExtent2D actualExtent = {
+            static_cast<uint32_t>(width),
+            static_cast<uint32_t>(height)
+        };
 
         actualExtent.width = std::max(capabilities.minImageExtent.width, std::min(capabilities.maxImageExtent.width, actualExtent.width));
         actualExtent.height = std::max(capabilities.minImageExtent.height, std::min(capabilities.maxImageExtent.height, actualExtent.height));
diff --git a/en/03_Drawing_a_triangle/04_Swap_chain_recreation.md b/en/03_Drawing_a_triangle/04_Swap_chain_recreation.md
@@ -114,25 +114,11 @@ Instead I've opted to clean up the existing command buffers with the
 `vkFreeCommandBuffers` function. This way we can reuse the existing pool to
 allocate the new command buffers.
 
-To handle window resizes properly, we also need to query the current size of the framebuffer to make sure that the swap chain images have the (new) right size. To do that change the `chooseSwapExtent` function to take the actual size into account:
-
-```c++
-VkExtent2D chooseSwapExtent(const VkSurfaceCapabilitiesKHR& capabilities) {
-    if (capabilities.currentExtent.width != UINT32_MAX) {
-        return capabilities.currentExtent;
-    } else {
-        int width, height;
-        glfwGetFramebufferSize(window, &width, &height);
-
-        VkExtent2D actualExtent = {
-            static_cast<uint32_t>(width),
-            static_cast<uint32_t>(height)
-        };
-
-        ...
-    }
-}
-```
+Note that in `chooseSwapExtent` we already query the new window resolution to
+make sure that the swap chain images have the (new) right size, so there's no
+need to modify `chooseSwapExtent` (remember that we already had to use
+`glfwGetFramebufferSize` get the resolution of the surface in pixel when
+creating the swap chain).
 
 That's all it takes to recreate the swap chain! However, the disadvantage of
 this approach is that we need to stop all rendering before creating the new swap
