GPU Rendering

While Skija performs exceptionally well on the CPU, its true power is unleashed when using GPU acceleration through OpenGL, Metal, Vulkan, or Direct3D.

Concepts

DirectContext

The DirectContext represents the connection to the GPU driver. It manages GPU resources, caches, and the state of the underlying graphics API.

BackendRenderTarget

A BackendRenderTarget represents a buffer (like an OpenGL Framebuffer or a Metal Texture) that Skia can draw into.

Example: OpenGL with LWJGL

To render to an OpenGL window using LWJGL:

1. Initialize OpenGL: Use LWJGL to create a window and a GL context.
2. Create DirectContext:

   DirectContext context = DirectContext.makeGL();

3. Create Render Target: You need to tell Skia which framebuffer to draw into (usually 0 for the default window).

   BackendRenderTarget rt = BackendRenderTarget.makeGL(
       width, height, 
       0, /* samples */ 
       8, /* stencil */ 
       fbId, /* GL framebuffer id */ 
       FramebufferFormat.GR_GL_RGBA8
   );

4. Create Surface:

   Surface surface = Surface.makeFromBackendRenderTarget(
       context, rt, 
       SurfaceOrigin.BOTTOM_LEFT, 
       ColorType.RGBA_8888, 
       ColorSpace.getSRGB()
   );

The Render Loop

In a GPU-accelerated environment, you must manually signal Skia to submit commands to the GPU.

while (!window.shouldClose()) {
    Canvas canvas = surface.getCanvas();
    
    // 1. Draw your content
    canvas.clear(0xFFFFFFFF);
    canvas.drawCircle(50, 50, 30, paint);
    
    // 2. Flush commands to the GPU
    context.flush();
    
    // 3. Swap buffers (graphics API specific)
    window.swapBuffers();
}

Best Practices

• Reuse the Context: Creating a DirectContext is expensive. Create it once and use it for the lifetime of your application.
• Handle Resizing: When the window is resized, you must close() the old Surface and BackendRenderTarget and create new ones that match the new dimensions.
• Resource Management: GPU-backed surfaces should be closed when no longer needed to free up VRAM.