Escher

Overview

Escher is a C++/Vulkan library that is used primarily by the Scenic implementation; it is not exposed by the Fuchsia SDK. It provides utilities for:

  1. Abstracting and optimizing simple operations with Vulkan, such as:

    • Pipeline reuse
    • Color correction with less memory bandwidth
      • Specification of sub-passes (and render-passes in general) is a bit more convenient in Escher than raw Vulkan, but it's pretty much the same idea, not significantly abstracted.
      • For example, Escher supports sub-passes within a single Vulkan render-pass. That is, when a second sub-pass needs to access a pixel at the same location, then it performs the transform inside the cached data without leaving the GPU memory hierarchy.
    • Updating buffers and sending draw commands directly
    • Resource lifetime management
      • For example, if you submit a Vulkan command-buffer which renders using a texture, and then delete the texture before rendering is complete, the result is undefined; a crash is likely. Escher doesn't delete the texture (or buffer) until it is no longer referenced by any active command buffers.

  2. Higher-level domain-specific renderers specific to Scenic, such as:

    a. RectangleCompositor, a simple compositor used by Flatland's VkRenderer to load display list contents into a single output image known as a framebuffer

    • RectangleCompositor leverages:
      • Flatland-specific Color Correction Vertex and Fragment Shaders, for use when Scenic performs color-correction using Vulkan
      • Flatland-specific Vertex and Fragment Shaders, for use when color correction is delegated to the display driver or not required at all
    • See the rainfall example for use of RectangleCompositor.

    b. PaperRenderer, a deprecated compositor used by Gfx which builds the abstraction for performing efficient 3D UX computations that were envisioned at the time. See the waterfall example for use of PaperRenderer.

Together, these libraries support rapid iteration and resource lifecycle management beyond what would be sustainably achievable when using Vulkan alone.

Cross-platform support

With a few exceptions, Escher is not platform-dependent. For example, the Escher examples can also be run on Linux. This provides two distinct benefits:

  • Faster iteration in development. Recompiling Escher and running to debug is much faster on Linux than on Fuchsia.
  • On other platforms, the Vulkan ecosystem provides a wide variety of development tools which are not available on Fuchsia. For example, the RenderDoc graphics debugger was very useful during the development of Escher.

GLSL support

Vulkan only accepts shader source code in the SPIR-V format. Escher optionally supports consuming GLSL, the human-readable format, then calls shaderc::Compiler to compile it to SPIR-V before loading into Vulkan.

Source Code

The source code lives in //src/ui/lib/escher/.