render engine example thanks to dougal2.

doc/python_api/examples/bpy.types.RenderEngine.py

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+"""
+Simple Render Engine
+++++++++++++++++++++
+"""
+
+import bpy
+
+
+class CustomRenderEngine(bpy.types.RenderEngine):
+    # These three members are used by blender to set up the
+    # RenderEngine; define its internal name, visible name and capabilities.
+    bl_idname = 'custom_renderer'
+    bl_label = 'Flat Color Renderer'
+    bl_use_preview = True
+
+    # This is the only method called by blender, in this example
+    # we use it to detect preview rendering and call the implementation
+    # in another method.
+    def render(self, scene):
+        scale = scene.render.resolution_percentage / 100.0
+        self.size_x = int(scene.render.resolution_x * scale)
+        self.size_y = int(scene.render.resolution_y * scale)
+
+        if scene.name == 'preview':
+            self.render_preview(scene)
+        else:
+            self.render_scene(scene)
+
+    # In this example, we fill the preview renders with a flat green color.
+    def render_preview(self, scene):
+        pixel_count = self.size_x * self.size_y
+
+        # The framebuffer is defined as a list of pixels, each pixel
+        # itself being a list of R,G,B,A values
+        green_rect = [[0.0, 1.0, 0.0, 1.0]] * pixel_count
+
+        # Here we write the pixel values to the RenderResult
+        result = self.begin_result(0, 0, self.size_x, self.size_y)
+        layer = result.layers[0]
+        layer.rect = green_rect
+        self.end_result(result)
+
+    # In this example, we fill the full renders with a flat blue color.
+    def render_scene(self, scene):
+        pixel_count = self.size_x * self.size_y
+
+        # The framebuffer is defined as a list of pixels, each pixel
+        # itself being a list of R,G,B,A values
+        blue_rect = [[0.0, 0.0, 1.0, 1.0]] * pixel_count
+
+        # Here we write the pixel values to the RenderResult
+        result = self.begin_result(0, 0, self.size_x, self.size_y)
+        layer = result.layers[0]
+        layer.rect = blue_rect
+        self.end_result(result)
+
+# Register the RenderEngine
+bpy.utils.register_class(CustomRenderEngine)
+
+# RenderEngines also need to tell UI Panels that they are compatible
+# Otherwise most of the UI will be empty when the engine is selected.
+# In this example, we need to see the main render image button and
+# the material preview panel.
+import properties_render
+properties_render.RENDER_PT_render.COMPAT_ENGINES.add('custom_renderer')
+del properties_render
+
+import properties_material
+properties_material.MATERIAL_PT_preview.COMPAT_ENGINES.add('custom_renderer')
+del properties_material