# Frame Capture with RenderDoc

[RenderDoc](https://renderdoc.org/) is a graphics debugger that can be used to capture frames. With Impeller starting to support OpenGL ES and Vulkan backends, RenderDoc can provide insights into the application's frames.

1. First step is to set up RenderDoc. Follow the [quickstart instructions](https://renderdoc.org/docs/getting_started/quick_start.html).
   For the purposes of this guide it is assumed that you are able to get RenderDoc running.
   If the RenderDoc installed from your package manager crashes on startup, consider [building from source](https://github.com/baldurk/renderdoc/blob/v1.x/docs/CONTRIBUTING/Compiling.md).
   
2. The next step would be to run the application you wish the capture the frames of.
   Typically these would be one of the [playground tests](https://github.com/flutter/engine/tree/main/impeller/playground),
   for example [those in entity_unittests.cc](https://github.com/flutter/engine/blob/main/impeller/entity/entity_unittests.cc).
   To build these, do:

   ```bash
   # In your $ENGINE_SRC folder, do:

   ./flutter/tools/gn --unopt
   ninja -C out/host_debug_unopt/
   ```

   Building a "debug_unopt" build ensures that you have tracing enabled. Without this, RenderDoc will not have much to show.

3. Start RenderDoc and (if necessary) select "Launch Application" button from the menu:

   ![Launch App](assets/renderdoc_frame_capture/launch-app.png)

   On Linux, the executable is `qrenderdoc`.

   You may also need to click the message that says "Click here to set up Vulkan capture".
   This will probably be needed if you built from source.

4. Fill out the configuration fields.
   Here, we will configure RenderDoc to specifically capture the "CanDrawRect" test:

   - executable path: `$ENGINE_SRC/out/host_debug/impeller_unittests` (expand `ENGINE_SRC`).
   - working directory: `$ENGINE_SRC` (expand `ENGINE_SRC`)
   - command-line arguments: `--gtest_filter="*CanDrawRect/Vulkan*" --enable_playground`

5. Click "Launch". If everything is working, you'll get a window with the selected unit test rendering,
   with a prompt in the top-left corner telling you to press `F12` or `Print Screen` to capture a frame.
   (If you do not, try capturing a different program, like factorio. On at least one occasion that has
   shaken things loose, though we have no explanation for why.)

   Press `ESC` to move on to the next test.

5. For the frame you wish to capture, press `F12`, you will now be able to see the frame capture and inspect the state:

   ![Renderdoc Capture](assets/renderdoc_frame_capture/render-doc-capture.png)

_See also:_

 * [Learning to Read GPU Frame Captures](https://github.com/flutter/engine/blob/main/impeller/docs/read_frame_captures.md)