Status

Not yet implemented. This document defines the contract for the OpenGL backend. Enable with make MOP_ENABLE_OPENGL=1 once implemented.

Location

src/backend/opengl/
  opengl_backend.c    — RHI function table implementation

Design

Context Creation

The OpenGL backend creates an offscreen rendering context:

• macOS: CGL pixel buffer context with kCGLOGLPVersion_3_2_Core
• Linux: EGL with EGL_OPENGL_API and pbuffer surface

No window or display server connection is required.

Framebuffer

An FBO with three attachments:

Shaders

Vertex shader:

• Inputs: aPosition (vec3), aNormal (vec3), aColor (vec4)
• Uniforms: uMVP (mat4), uModel (mat4)
• Outputs: vNormal (vec3), vColor (vec4)

Fragment shader:

• Computes flat shading: ambient + diffuse * max(N·L, 0)
• Writes fragColor (vec4) to color 0
• Writes fragObjectId (uint) to color 1

Draw Call

1. Bind VAO with vertex buffer
2. Upload uMVP, uModel, uObjectId uniforms
3. glDrawElements(GL_TRIANGLES, ...)

Readback

glReadPixels from the FBO. Color is read as GL_RGBA / GL_UNSIGNED_BYTE. Object ID is read as GL_RED_INTEGER / GL_UNSIGNED_INT. Depth is read as GL_DEPTH_COMPONENT / GL_FLOAT.

RHI Handle Mapping