Create missing directories before saving a PNG

docs/headless.md

@@ -83,9 +83,9 @@ builder.UseDefaultRendering(new WindowConfig(Size: (1280, 720), Title: "Hotbar")
 
 An agent usually needs no code change for this: `PIXELY_HEADLESS=1` switches any app to headless mode, see [Environment variables](#environment-variables).
 
-`screenshot <path>` writes the last rendered frame as a PNG; the file exists once the command's frame has run. A path that cannot be written, or a screenshot before the first frame has rendered, throws out of the frame loop. Commands run before that frame's render, so a screenshot right after the commands it should show is one frame too early; put `wait 1;` before it. The capture waits for the GPU, so that frame takes longer.
+`screenshot <path>` writes the last rendered frame as a PNG; the file exists once the command's frame has run. Missing directories of the path are created. A path that still cannot be written, such as one whose parent is a file, throws out of the frame loop. So does a screenshot before the first frame has rendered. Commands run before that frame's render, so a screenshot right after the commands it should show is one frame too early; put `wait 1;` before it. The capture waits for the GPU, so that frame takes longer.
 
-The frame is also available to code through `OffscreenWindow.CaptureLastFrame()`, and `IImageWriter` saves a tightly packed, non-planar, non-indexed `Image` as a PNG; `Build()` registers the SDL one unless the app registered its own.
+The frame is also available to code through `OffscreenWindow.CaptureLastFrame()`, and `IImageWriter` saves a tightly packed, non-planar, non-indexed `Image` as a PNG, creating missing directories; `Build()` registers the SDL one unless the app registered its own.
 
 ### Environment variables
 

src/Pixely/Content/SdlImageWriter.cs

@@ -20,6 +20,12 @@ public unsafe void SavePng(Image image, string path)
             throw new NotSupportedException($"Only tightly packed images in a non-planar, non-indexed pixel format can be saved, not {image.PixelFormat} with {pixels.Length} bytes for {width}x{height}.");
         }
 
+        string? directory = Path.GetDirectoryName(Path.GetFullPath(path));
+        if (directory != null)
+        {
+            Directory.CreateDirectory(directory);
+        }
+
         fixed (byte* pixelsPointer = pixels)
         {
             Pointer<SDL_Surface> surface = SDL3.SDL_CreateSurfaceFrom(width, height, format, (IntPtr)pixelsPointer, pitch);

tests/Pixely.Tests/SdlImageWriterTests.cs

@@ -0,0 +1,42 @@
+using Pixely.Content;
+using Pixely.Gpu;
+
+namespace Pixely.Tests;
+
+public class SdlImageWriterTests
+{
+    private DirectoryInfo _temporaryDirectory = null!;
+
+    [SetUp]
+    public void SetUp()
+    {
+        _temporaryDirectory = Directory.CreateTempSubdirectory("Pixely.Tests-");
+    }
+
+    [TearDown]
+    public void TearDown()
+    {
+        _temporaryDirectory.Delete(recursive: true);
+    }
+
+    [Test]
+    public void SavePng_CreatesMissingDirectories()
+    {
+        string path = Path.Combine(_temporaryDirectory.FullName, "frames", "run", "shot.png");
+
+        new SdlImageWriter().SavePng(CreateImage(), path);
+
+        Assert.That(File.Exists(path), Is.True);
+    }
+
+    [Test]
+    public void SavePng_WithParentThatIsAFile_Throws()
+    {
+        string parent = Path.Combine(_temporaryDirectory.FullName, "frames");
+        File.WriteAllBytes(parent, []);
+
+        Assert.That(() => new SdlImageWriter().SavePng(CreateImage(), Path.Combine(parent, "shot.png")), Throws.InstanceOf<IOException>());
+    }
+
+    private static RawImage CreateImage() => new(new byte[2 * 2 * 4], new ShortSize(2, 2), PixelFormat.Rgba8888);
+}