Rust 当我每次渲染屏幕时不调用`graphics：：clear`时，如何阻止 screen flash

我认为闪烁是由缓冲区交换引起的:在flashes.rs中，只有第一个要被抽取的缓冲区被清除.第二个是全零，如果你运气不好的话，也可以是gpu剩余的内存.根据OpenGL wiki，拨打graphics::clear没有什么好办法:

现代OpenGL程序应始终使用双缓冲.

相反，通常的方法是累积对纹理或renderbuffer的更改，然后将其绘制到屏幕上，就像您描述的那样.

我也找不到任何方法在opengl_graphics以内实现这一点(其中任何地方都没有gl::GenFramebuffers的呼叫)，但使用原始gl呼叫进行设置相对简单.(我使用纹理而不是渲染缓冲区，因为它们有一个显著的优势，即可以得到Image::draw等高级方法的支持.)

extern crate piston;
extern crate opengl_graphics;
extern crate graphics;
extern crate sdl2_window;
extern crate gl;

use opengl_graphics::{ GlGraphics, OpenGL, Texture, TextureSettings };
use graphics::{ Context, Graphics, Transformed };
use graphics::image::Image;
use piston::input::*;
use piston::event_loop::*;
use piston::window::Window;
use sdl2_window::Sdl2Window as AppWindow;
use gl::types::GLuint;

static CURSOR_POS_COLOR: [f32; 4] = [0.0, 0.0, 0.0, 1.0];

fn main() {
    let opengl = OpenGL::V3_2;
    let mut window: AppWindow = piston::window::WindowSettings::new("Example for StackOverflow", [600, 600])
        .exit_on_esc(true).opengl(opengl).build().expect("window");

    let ref mut gl = GlGraphics::new(opengl);
    let (mut mx, mut my) = (0., 0.);

    let draw_size = window.draw_size();
    // It would also be possible to create a texture by hand using gl::GenTextures and call
    // gl::TexImage2D with a null pointer for the data argument, which would require another unsafe
    // block but would save this allocation
    let texture_buf = vec![0u8; draw_size.width as usize * draw_size.height as usize];
    let texture = Texture::from_memory_alpha(&texture_buf, draw_size.width, draw_size.height,
                                             &TextureSettings::new()).expect("texture");

    let fbo;
    unsafe {
        let mut fbos: [GLuint; 1] = [0];
        // Create a Framebuffer Object that we can draw to later
        gl::GenFramebuffers(1, fbos.as_mut_ptr());
        fbo = fbos[0];
        // Switch to it as the active framebuffer
        gl::BindFramebuffer(gl::FRAMEBUFFER, fbo);
        // Set up the framebuffer object so that draws to it will go to the texture
        gl::FramebufferTexture2D(gl::FRAMEBUFFER,
                                 gl::COLOR_ATTACHMENT0, // draw colors, not depth or stencil data
                                 gl::TEXTURE_2D, // the texture's type
                                 texture.get_id(),
                                 0); // mipmap level
    }

    let mut events = Events::new(EventSettings::new().lazy(true));
    while let Some(e) = events.next(&mut window) {
        e.mouse_cursor(|x, y| {
            mx = x; my = y;
        });
        e.render(|args| {
            // Switch to the texture framebuffer and draw the cursor
            unsafe {
                gl::BindFramebuffer(gl::FRAMEBUFFER, fbo);
            }
            gl.draw(args.viewport(), |c, g| {
                draw_cursor_pos([mx, my], &c, g);
            });

            // Switch to the window framebuffer and draw the texture
            unsafe {
                gl::BindFramebuffer(gl::FRAMEBUFFER, 0);
            }
            gl.draw(args.viewport(), |c, g| {
                graphics::clear([1f32, 1f32, 1f32, 0f32], g);
                // I can't entirely explain this.  We already applied the viewport transform when
                // we were rendering the cursor, so I think the texture is right-side-up for GL,
                // but piston::Image is expecting an image laid out in screen coordinates.
                // Since there is an offset in the viewport transform, the flip has to be applied
                // first, otherwise it would flip across the origin.
                let flipped = c.transform.prepend_transform(graphics::math::scale(1., -1.));
                Image::new().draw(&texture, &c.draw_state, flipped, g);
            });
        });
    }
}

fn draw_cursor_pos<G: Graphics>(
    cursor: [f64; 2],
    c: &Context,
    g: &mut G,
) {
    graphics::ellipse(
        CURSOR_POS_COLOR,
        graphics::ellipse::circle(cursor[0], cursor[1], 4.0),
        c.transform,
        g
    );
}

或者，gfx后端有一种听起来很有前途的Factory::CreateRenderTarget方法.我的硬件不支持它，但我相信使用它会大致如下:

extern crate piston;
extern crate graphics;
extern crate piston_window;
extern crate gfx_core;

use graphics::{ Context, Graphics, Transformed };
use graphics::image::Image;
use piston::input::*;
use piston::event_loop::*;
use piston::window::Window;
use piston_window::{ PistonWindow, OpenGL, G2dTexture };
use gfx_core::factory::Factory;
use gfx_core::texture::{ SamplerInfo, FilterMethod, WrapMode, Size };

static CURSOR_POS_COLOR: [f32; 4] = [0.0, 0.0, 0.0, 1.0];

fn main() {
    let opengl = OpenGL::V2_1;
    let window_settings =
        piston::window::WindowSettings::new("Example for StackOverflow", [600, 600])
        .opengl(opengl)
        .exit_on_esc(true);
    let mut window: PistonWindow = window_settings.build().expect("window");
    window.set_lazy(true);

    let size = window.draw_size();

    let (texture_handle, shader_view, target) = window.factory.create_render_target(size.width as Size, size.height as Size)
        .expect("render target");
    let sampler = window.factory.create_sampler(SamplerInfo::new(FilterMethod::Scale, WrapMode::Tile));
    let texture = G2dTexture {
        surface: texture_handle,
        sampler: sampler,
        view: shader_view,
    };
    let stencil = window.factory.create_depth_stencil_view_only(size.width as Size, size.height as Size)
        .expect("stencil");

    let (mut mx, mut my) = (0., 0.);

    while let Some(e) = window.next() {
        e.mouse_cursor(|x, y| {
            mx = x; my = y;
        });
        if let Some(args) = e.render_args() {
            window.g2d.draw(&mut window.encoder, &target, &stencil, args.viewport(), |c, g| {
                draw_cursor_pos([mx, my], &c, g);
            });

            window.draw_2d(&e, |c, g| {
                graphics::clear([1f32, 1f32, 1f32, 0f32], g);
                let flipped = c.transform.prepend_transform(graphics::math::scale(1., -1.));
                Image::new().draw(&texture, &c.draw_state, flipped, g);
            });
        }
    }
}

fn draw_cursor_pos<G: Graphics>(
    cursor: [f64; 2],
    c: &Context,
    g: &mut G,
) {
    graphics::ellipse(
        CURSOR_POS_COLOR,
        graphics::ellipse::circle(cursor[0], cursor[1], 4.0),
        c.transform,
        g
    );
}