Rendering with transparency over previous frames

[guija]

Hi, I would like to achieve the following, but I'm not sure how to do it and I also don't know exactly what to search for. I have a scene and I want to re-render it every time, but overlay the new rendering with the last e.g. 5 frames. E.g. to create the effect of a streak when e.g. a sphere is moving. I read that this can be done using "blending and framebuffer accumulation", but I don't know which mechanisms to use in three_d. I played around with ClearState values, but that doesn't have the desired effect. This is the way I'm currently rendering my scene:

frame_input
            .screen()
            .clear(ClearState::color_and_depth(0.0, 0.0, 0.0, 1.0, 1.0))
            .render(
                &camera,
                objects_to_render_for_main.into_iter(),
                &[&sun_light, &ambient_light],
            );

I would like to try to e.g. just clone the current frame, keep it e.g. a Vec and then just rendering the current frame, make it a bit transparent and then mix the colors and write this to the actual screen / render target. However I don't know how to...

• ... render the current frame a kind of "frame buffer" without showing it.
• ... access the raw data of the current frame so that I can access it in the next frame calculation.
• ... just copy rgba values to the output screen instead of using the rendering mechanisms.

Can someone give hints with mechanisms / functions to use? Thanks a lot!|

[asny]

That's a very advanced use-case, my advice would be to do something simpler to learn the graphics techniques involved. However;

... render the current frame a kind of "frame buffer" without showing it.

It's called a RenderTarget in three-d, or if you just need the colors and no depth sorting, a ColorTarget. Just create a new one of those and render into it instead of the frame_input.screen().

... access the raw data of the current frame so that I can access it in the next frame calculation.

You could read out the data to the CPU using RenderTarget::read_color but that would be terrible for performance. Instead you can use the Texture2D used to create the ColorTarget as a texture to read from when blending the results in the final pass.

... just copy rgba values to the output screen instead of using the rendering mechanisms.

CopyEffect or ScreenEffect

[guija]

Thanks for the hints. I will try this.