Fill the hello window with an RGB gradient

Red grows to the right, green downward, and blue fades as red grows.
Rows now step by the frame's stride_bytes instead of assuming packed
pixels.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

2 files changed, 12 insertions, 5 deletions

diff --git a/docs/getting-started.md b/docs/getting-started.md
index 906d425..73af4ab 100644
--- a/docs/getting-started.md
+++ b/docs/getting-started.md
@@ -39,8 +39,8 @@ CPU pixel targets are presented through the system's Metal device; applications
 do not need to create or manage that device themselves.
 
 The complete beginner example is [hello.c](../examples/hello.c). It lets
-`yasl_run()` create its window from `yasl_app_desc.window`, fills every pixel,
-and presents inside a frame callback.
+`yasl_run()` create its window from `yasl_app_desc.window`, fills every pixel
+with an RGB gradient, and presents inside a frame callback.
 `yasl_run()` handles input delivery, display notifications, and cleanup. The
 [interactive window](../examples/simple_window.c) adds pointer input; the
 [tablet example](../examples/tablet.c) demonstrates retained application content,
diff --git a/examples/hello.c b/examples/hello.c
index 38a0cac..4c171ec 100644
--- a/examples/hello.c
+++ b/examples/hello.c
@@ -6,9 +6,16 @@ static bool failed;
 static void draw(void *ud, struct yasl_surface *surface, struct yasl_frame_context *frame)
 {
     (void)ud;
-    size_t count = (size_t)frame->size.x * frame->size.y;
-    for (size_t i = 0; i < count; i++)
-        frame->cpu.pixels[i] = 0xff306090; /* Opaque premultiplied ARGB. */
+    int w = frame->size.x, h = frame->size.y;
+    for (int y = 0; y < h; y++) {
+        uint32_t *row = frame->cpu.pixels + (size_t)y * (frame->cpu.stride_bytes / 4);
+        for (int x = 0; x < w; x++) {
+            /* Red grows rightward, green downward, blue fades; opaque premultiplied ARGB. */
+            uint32_t r = (uint32_t)(x * 255 / (w > 1 ? w - 1 : 1));
+            uint32_t g = (uint32_t)(y * 255 / (h > 1 ? h - 1 : 1));
+            row[x] = 0xff000000 | r << 16 | g << 8 | (255 - r);
+        }
+    }
 
     if (!yasl_frame_present_cpu(frame, NULL, 0)) {
         fprintf(stderr, "Could not present the draw target\n");