Refactor: add gpu_create_post_process_texture helper

Adds new helper for common post-process texture pattern (RenderAttachment
| TextureBinding | CopySrc usage). Refactors test_post_process_helper.cc
to use gpu_create_buffer() and gpu_create_post_process_texture(),
eliminating 91 lines of boilerplate.

- New: gpu_create_post_process_texture() in gpu.{h,cc}
- Refactor: test_post_process_helper.cc uses helpers instead of raw WGPU
- Doc: Updated WGPU_HELPERS.md with usage examples
- Verified: All tests passing (test_post_process_helper, test_demo_effects)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

4 files changed, 95 insertions, 90 deletions

diff --git a/doc/WGPU_HELPERS.md b/doc/WGPU_HELPERS.md
index 5642eef..f1d83d5 100644
--- a/doc/WGPU_HELPERS.md
+++ b/doc/WGPU_HELPERS.md
@@ -133,6 +133,45 @@ TextureWithView output = gpu_create_storage_texture_2d(
 
 ---
 
+#### `gpu_create_post_process_texture`
+```cpp
+TextureWithView gpu_create_post_process_texture(
+    WGPUDevice device, uint32_t width, uint32_t height,
+    WGPUTextureFormat format);
+```
+
+**Purpose:** Create texture for post-processing effects (render target + input + readback).
+
+**Parameters:**
+- `device` - WGPU device handle
+- `width`, `height` - Texture dimensions
+- `format` - Pixel format (typically `WGPUTextureFormat_RGBA8Unorm`)
+
+**Returns:** `TextureWithView` with usage flags:
+- `WGPUTextureUsage_RenderAttachment` - Can render to it
+- `WGPUTextureUsage_TextureBinding` - Can sample from it
+- `WGPUTextureUsage_CopySrc` - Can read back pixels
+
+**Example:**
+```cpp
+// Create ping-pong buffers for multi-pass post-processing
+TextureWithView buffer_a = gpu_create_post_process_texture(
+    device, 1920, 1080, WGPUTextureFormat_RGBA8Unorm);
+TextureWithView buffer_b = gpu_create_post_process_texture(
+    device, 1920, 1080, WGPUTextureFormat_RGBA8Unorm);
+
+// Pass 1: Render to buffer_a
+// Pass 2: Sample buffer_a, render to buffer_b
+// Pass 3: Copy buffer_b to screen
+```
+
+**Common Use Cases:**
+- Post-process ping-pong buffers
+- Off-screen render targets that need sampling
+- Test fixtures requiring texture readback
+
+---
+
 #### `gpu_create_mip_view`
 ```cpp
 WGPUTextureView gpu_create_mip_view(WGPUTexture texture,
diff --git a/src/gpu/gpu.cc b/src/gpu/gpu.cc
index 535de67..afbba90 100644
--- a/src/gpu/gpu.cc
+++ b/src/gpu/gpu.cc
@@ -84,6 +84,16 @@ TextureWithView gpu_create_storage_texture_2d(WGPUDevice device, uint32_t width,
       WGPUTextureUsage_StorageBinding | WGPUTextureUsage_TextureBinding, 1);
 }
 
+TextureWithView gpu_create_post_process_texture(WGPUDevice device,
+                                                uint32_t width, uint32_t height,
+                                                WGPUTextureFormat format) {
+  return gpu_create_texture_2d(
+      device, width, height, format,
+      WGPUTextureUsage_RenderAttachment | WGPUTextureUsage_TextureBinding |
+          WGPUTextureUsage_CopySrc,
+      1);
+}
+
 WGPUTextureView gpu_create_mip_view(WGPUTexture texture,
                                     WGPUTextureFormat format,
                                     uint32_t mip_level) {
diff --git a/src/gpu/gpu.h b/src/gpu/gpu.h
index 74e0eb7..0b0153b 100644
--- a/src/gpu/gpu.h
+++ b/src/gpu/gpu.h
@@ -97,6 +97,9 @@ TextureWithView gpu_create_texture_2d(WGPUDevice device, uint32_t width,
 TextureWithView gpu_create_storage_texture_2d(WGPUDevice device, uint32_t width,
                                               uint32_t height,
                                               WGPUTextureFormat format);
+TextureWithView gpu_create_post_process_texture(WGPUDevice device,
+                                                uint32_t width, uint32_t height,
+                                                WGPUTextureFormat format);
 WGPUTextureView gpu_create_mip_view(WGPUTexture texture,
                                     WGPUTextureFormat format, uint32_t mip_level);
 WGPUTextureView gpu_create_texture_view_2d(WGPUTexture texture,
diff --git a/src/tests/gpu/test_post_process_helper.cc b/src/tests/gpu/test_post_process_helper.cc
index 08b9e49..42b5d79 100644
--- a/src/tests/gpu/test_post_process_helper.cc
+++ b/src/tests/gpu/test_post_process_helper.cc
@@ -18,28 +18,7 @@ extern void pp_update_bind_group(WGPUDevice device, WGPURenderPipeline pipeline,
                                  WGPUTextureView input_view,
                                  GpuBuffer uniforms, GpuBuffer effect_params);
 
-// Helper: Create a texture suitable for post-processing (both render target and
-// texture binding)
-static WGPUTexture create_post_process_texture(WGPUDevice device, int width,
-                                               int height,
-                                               WGPUTextureFormat format) {
-  const WGPUTextureDescriptor texture_desc = {
-      .usage = WGPUTextureUsage_RenderAttachment |
-               WGPUTextureUsage_TextureBinding | WGPUTextureUsage_CopySrc,
-      .dimension = WGPUTextureDimension_2D,
-      .size = {static_cast<uint32_t>(width), static_cast<uint32_t>(height), 1},
-      .format = format,
-      .mipLevelCount = 1,
-      .sampleCount = 1,
-  };
-  return wgpuDeviceCreateTexture(device, &texture_desc);
-}
-
-// Helper: Create texture view
-static WGPUTextureView create_texture_view(WGPUTexture texture,
-                                           WGPUTextureFormat format) {
-  return gpu_create_texture_view_2d(texture, format);
-}
+// Helpers are now in gpu.h (gpu_create_post_process_texture, gpu_create_texture_view_2d)
 
 // Minimal valid post-process shader for testing
 static const char* test_shader = R"(
@@ -87,30 +66,21 @@ static void test_bind_group_creation(WebGPUTestFixture& fixture) {
   assert(pipeline != nullptr && "Pipeline required for bind group test");
 
   // Create input texture with TEXTURE_BINDING usage
-  WGPUTexture input_texture =
-      create_post_process_texture(fixture.device(), 256, 256, fixture.format());
-  WGPUTextureView input_view =
-      create_texture_view(input_texture, fixture.format());
-
-  // Create uniform buffer
-  const WGPUBufferDescriptor uniform_desc = {
-      .usage = WGPUBufferUsage_Uniform | WGPUBufferUsage_CopyDst,
-      .size = 16, // vec4<f32>
-  };
-  WGPUBuffer uniform_buffer =
-      wgpuDeviceCreateBuffer(fixture.device(), &uniform_desc);
-  assert(uniform_buffer != nullptr && "Uniform buffer should be created");
+  TextureWithView input =
+      gpu_create_post_process_texture(fixture.device(), 256, 256, fixture.format());
 
-  GpuBuffer uniforms = {uniform_buffer, 16};
+  // Create uniform buffers using gpu_create_buffer
+  GpuBuffer uniforms = gpu_create_buffer(
+      fixture.device(), 16, WGPUBufferUsage_Uniform | WGPUBufferUsage_CopyDst);
+  assert(uniforms.buffer != nullptr && "Uniform buffer should be created");
 
   // Dummy effect params buffer for testing (matches vec4<f32>)
-  WGPUBuffer dummy_params_buffer_handle =
-      wgpuDeviceCreateBuffer(fixture.device(), &uniform_desc);
-  GpuBuffer dummy_effect_params_buffer = {dummy_params_buffer_handle, 16};
+  GpuBuffer dummy_effect_params_buffer = gpu_create_buffer(
+      fixture.device(), 16, WGPUBufferUsage_Uniform | WGPUBufferUsage_CopyDst);
 
   // Test bind group creation
   WGPUBindGroup bind_group = nullptr;
-  pp_update_bind_group(fixture.device(), pipeline, &bind_group, input_view,
+  pp_update_bind_group(fixture.device(), pipeline, &bind_group, input.view,
                        uniforms, dummy_effect_params_buffer);
 
   assert(bind_group != nullptr && "Bind group should be created successfully");
@@ -118,10 +88,10 @@ static void test_bind_group_creation(WebGPUTestFixture& fixture) {
 
   // Cleanup
   wgpuBindGroupRelease(bind_group);
-  wgpuTextureViewRelease(input_view);
-  wgpuTextureRelease(input_texture);
-  wgpuBufferRelease(uniform_buffer);
-  wgpuBufferRelease(dummy_params_buffer_handle);
+  wgpuTextureViewRelease(input.view);
+  wgpuTextureRelease(input.texture);
+  wgpuBufferRelease(uniforms.buffer);
+  wgpuBufferRelease(dummy_effect_params_buffer.buffer);
   wgpuRenderPipelineRelease(pipeline);
   fprintf(stdout, "  ✓ Resources released\n");
 }
@@ -133,48 +103,39 @@ static void test_bind_group_update(WebGPUTestFixture& fixture) {
   WGPURenderPipeline pipeline = create_post_process_pipeline(
       fixture.device(), fixture.format(), test_shader);
 
-  WGPUTexture texture1 =
-      create_post_process_texture(fixture.device(), 256, 256, fixture.format());
-  WGPUTextureView view1 = create_texture_view(texture1, fixture.format());
+  TextureWithView texture1 =
+      gpu_create_post_process_texture(fixture.device(), 256, 256, fixture.format());
+  TextureWithView texture2 =
+      gpu_create_post_process_texture(fixture.device(), 512, 512, fixture.format());
 
-  WGPUTexture texture2 =
-      create_post_process_texture(fixture.device(), 512, 512, fixture.format());
-  WGPUTextureView view2 = create_texture_view(texture2, fixture.format());
-
-  const WGPUBufferDescriptor uniform_desc = {
-      .usage = WGPUBufferUsage_Uniform | WGPUBufferUsage_CopyDst,
-      .size = 16,
-  };
-  WGPUBuffer uniform_buffer =
-      wgpuDeviceCreateBuffer(fixture.device(), &uniform_desc);
-  GpuBuffer uniforms = {uniform_buffer, 16};
+  GpuBuffer uniforms = gpu_create_buffer(
+      fixture.device(), 16, WGPUBufferUsage_Uniform | WGPUBufferUsage_CopyDst);
 
   // Dummy effect params buffer for testing (matches vec4<f32>)
-  WGPUBuffer dummy_params_buffer_handle =
-      wgpuDeviceCreateBuffer(fixture.device(), &uniform_desc);
-  GpuBuffer dummy_effect_params_buffer = {dummy_params_buffer_handle, 16};
+  GpuBuffer dummy_effect_params_buffer = gpu_create_buffer(
+      fixture.device(), 16, WGPUBufferUsage_Uniform | WGPUBufferUsage_CopyDst);
 
   // Create initial bind group
   WGPUBindGroup bind_group = nullptr;
-  pp_update_bind_group(fixture.device(), pipeline, &bind_group, view1, uniforms,
-                       dummy_effect_params_buffer);
+  pp_update_bind_group(fixture.device(), pipeline, &bind_group, texture1.view,
+                       uniforms, dummy_effect_params_buffer);
   assert(bind_group != nullptr && "Initial bind group should be created");
   fprintf(stdout, "  ✓ Initial bind group created\n");
 
   // Update bind group (should release old and create new)
-  pp_update_bind_group(fixture.device(), pipeline, &bind_group, view2, uniforms,
-                       dummy_effect_params_buffer);
+  pp_update_bind_group(fixture.device(), pipeline, &bind_group, texture2.view,
+                       uniforms, dummy_effect_params_buffer);
   assert(bind_group != nullptr && "Updated bind group should be created");
   fprintf(stdout, "  ✓ Bind group updated successfully\n");
 
   // Cleanup
   wgpuBindGroupRelease(bind_group);
-  wgpuTextureViewRelease(view1);
-  wgpuTextureRelease(texture1);
-  wgpuTextureViewRelease(view2);
-  wgpuTextureRelease(texture2);
-  wgpuBufferRelease(uniform_buffer);
-  wgpuBufferRelease(dummy_params_buffer_handle);
+  wgpuTextureViewRelease(texture1.view);
+  wgpuTextureRelease(texture1.texture);
+  wgpuTextureViewRelease(texture2.view);
+  wgpuTextureRelease(texture2.texture);
+  wgpuBufferRelease(uniforms.buffer);
+  wgpuBufferRelease(dummy_effect_params_buffer.buffer);
   wgpuRenderPipelineRelease(pipeline);
   fprintf(stdout, "  ✓ Resources released\n");
 }
@@ -189,31 +150,23 @@ static void test_full_setup(WebGPUTestFixture& fixture) {
   assert(pipeline != nullptr && "Pipeline creation failed");
 
   // Create input texture (with TEXTURE_BINDING usage)
-  WGPUTexture input_texture =
-      create_post_process_texture(fixture.device(), 256, 256, fixture.format());
-  WGPUTextureView input_view =
-      create_texture_view(input_texture, fixture.format());
+  TextureWithView input =
+      gpu_create_post_process_texture(fixture.device(), 256, 256, fixture.format());
 
   // Create output texture (can use OffscreenRenderTarget for this)
   OffscreenRenderTarget output_target(fixture.instance(), fixture.device(), 256,
                                       256);
 
-  const WGPUBufferDescriptor uniform_desc = {
-      .usage = WGPUBufferUsage_Uniform | WGPUBufferUsage_CopyDst,
-      .size = 16,
-  };
-  WGPUBuffer uniform_buffer =
-      wgpuDeviceCreateBuffer(fixture.device(), &uniform_desc);
-  GpuBuffer uniforms = {uniform_buffer, 16};
+  GpuBuffer uniforms = gpu_create_buffer(
+      fixture.device(), 16, WGPUBufferUsage_Uniform | WGPUBufferUsage_CopyDst);
 
   // Dummy effect params buffer for testing (matches vec4<f32>)
-  WGPUBuffer dummy_params_buffer_handle =
-      wgpuDeviceCreateBuffer(fixture.device(), &uniform_desc);
-  GpuBuffer dummy_effect_params_buffer = {dummy_params_buffer_handle, 16};
+  GpuBuffer dummy_effect_params_buffer = gpu_create_buffer(
+      fixture.device(), 16, WGPUBufferUsage_Uniform | WGPUBufferUsage_CopyDst);
 
   // Create bind group
   WGPUBindGroup bind_group = nullptr;
-  pp_update_bind_group(fixture.device(), pipeline, &bind_group, input_view,
+  pp_update_bind_group(fixture.device(), pipeline, &bind_group, input.view,
                        uniforms, dummy_effect_params_buffer);
   assert(bind_group != nullptr && "Bind group creation failed");
 
@@ -252,10 +205,10 @@ static void test_full_setup(WebGPUTestFixture& fixture) {
   wgpuRenderPassEncoderRelease(pass);
   wgpuCommandEncoderRelease(encoder);
   wgpuBindGroupRelease(bind_group);
-  wgpuTextureViewRelease(input_view);
-  wgpuTextureRelease(input_texture);
-  wgpuBufferRelease(uniform_buffer);
-  wgpuBufferRelease(dummy_params_buffer_handle);
+  wgpuTextureViewRelease(input.view);
+  wgpuTextureRelease(input.texture);
+  wgpuBufferRelease(uniforms.buffer);
+  wgpuBufferRelease(dummy_effect_params_buffer.buffer);
   wgpuRenderPipelineRelease(pipeline);
 
   fprintf(stdout, "  ✓ Full setup test completed\n");