目标：

有效地读取和写入相同的纹理，就像Shadertoy处理缓冲区一样。

设置：

我有一个基本的反馈系统，有两个纹理，每个纹理都连接到一个帧缓冲区。当我渲染到帧缓冲区1时，我绑定纹理2以在着色器中采样。然后，当我渲染到帧缓冲区2时，我绑定纹理1以在着色器中采样，并重复。最后，我用默认的帧缓冲区和sperate着色器将纹理1输出到整个屏幕。

问题：

这几乎可以像预期的那样工作，因为我可以从着色器中的纹理读取，也可以输出到着色器，创建所需的反馈循环。
问题是，帧缓冲区似乎没有完全清除到黑色。
为了测试，我做了一个简单的拖尾效果。
在shadertoy中，轨迹完全按预期消失：

Live in shadertoy
但在我的应用程序中，痕迹开始消失，但留下了少量：

我的想法是我没有正确地清除帧缓冲区，或者我没有在这个例子中正确地使用GLFW的双缓冲。我已经尝试了清除帧缓冲区的每一种组合，但我肯定在这里遗漏了一些东西。

代码：

这是一个带有移动圆圈的拖尾效果着色器（与上图相同）

# version 330

precision highp float;

uniform sampler2D samplerA; // Texture sampler

uniform float uTime; // current execution time
uniform vec2 uResolution; // resolution of window

void main()
{
    vec2 uv = gl_FragCoord.xy / uResolution.xy; // Coordinates from 0 - 1
    vec3 tex = texture(samplerA, uv).xyz;// Read ping pong texture that we are writing to 

    vec2 pos = .3*vec2(cos(uTime), sin(uTime)); // Circle position (circular motion around screen)
    vec3 c = mix(vec3(1.), vec3(0), step(.0, length(uv - pos)-.07)); // Circle color

    tex = mix(c, tex, .981); // Replace some circle color with the texture color

    gl_FragColor = vec4(tex, 1.0); // Output to texture
}

帧缓冲区和纹理创建：

// -- Generate frame buffer 1 --
glGenFramebuffers(1, &frameBuffer1);
glBindFramebuffer(GL_FRAMEBUFFER, frameBuffer1);

// Generate texture 1
glGenTextures(1, &texture1);

// Bind the newly created texture
glBindTexture(GL_TEXTURE_2D, texture1);

// Create an empty image
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 1920, 1080, 0, GL_RGBA, GL_FLOAT, 0);

// Nearest filtering, for sampling
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);

// Attach output texture to frame buffer
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texture1, 0);

// -- Generate frame buffer 2 --
glGenFramebuffers(1, &frameBuffer2);
glBindFramebuffer(GL_FRAMEBUFFER, frameBuffer2);

// Generate texture 2
glGenTextures(1, &texture2);

// Bind the newly created texture
glBindTexture(GL_TEXTURE_2D, texture2);

// Create an empty image
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 1920, 1080, 0, GL_RGBA, GL_FLOAT, 0);

// Nearest filtering, for sampling
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);

// Attach texture 2 to frame buffer 2
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texture2, 0);

主回路：

while(programIsRunning){

// Draw scene twice, once to frame buffer 1 and once to frame buffer 2 
for (int i = 0; i < 2; i++)
{
    // Start trailing effect shader program
    glUseProgram(program);

    glViewport(0, 0, platform.windowWidth(), platform.windowHeight());

    // Write to frame buffer 1
    if (i == 0)
    {
        // Bind and clear frame buffer 1
        glBindFramebuffer(GL_FRAMEBUFFER, frameBuffer1); 
        glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);

        // Bind texture 2 for sampler
        glActiveTexture(GL_TEXTURE0 + 0);
        glBindTexture(GL_TEXTURE_2D, texture2); 
        glUniform1i(uniforms.samplerA, 0);
    }
    else // Write to frame buffer 2
    {
        // Bind and clear frame buffer 2
        glBindFramebuffer(GL_FRAMEBUFFER, frameBuffer2);
        glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);

        // Bind texture 1 for sampler
        glActiveTexture(GL_TEXTURE0 + 0);
        glBindTexture(GL_TEXTURE_2D, texture1);
        glUniform1i(uniforms.samplerA, 0);
    }

    // Render to screen
    glDrawArrays(GL_TRIANGLES, 0, 6);
}

// Start screen shader program
glUseProgram(screenProgram);

// Bind default frame buffer
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);

glViewport(0, 0, platform.windowWidth(), platform.windowHeight());

// Bind texture 1 for sampler (binding texture 2 should be the same?)
glActiveTexture(GL_TEXTURE0 + 0);
glBindTexture(GL_TEXTURE_2D, texture1);
glUniform1i(uniforms.samplerA, 0);

// Draw final rectangle to screen
glDrawArrays(GL_TRIANGLES, 0, 6);

// Swap glfw buffers
glfwSwapBuffers(platform.window());

}

如果这是一个清除的问题，我真的很想知道为什么。改变哪个帧缓冲区被清除似乎没有改变任何事情。我会继续试验在此期间。谢谢！