如何克服屏幕外和屏幕内帧缓冲渲染之间的差异？

问：

我正在尝试使用屏幕外的帧缓冲区来复制一个渲染效果出色的场景到默认帧缓冲区。渲染中似乎存在我无法理清的差异。

作为上下文，我正在使用大气着色器可视化地球。我使用的是 QT QOpenGLWidget，但主要是原始 GL 调用，因为我不喜欢 QT 的抽象。我需要将此场景渲染到屏幕外的帧缓冲区，因为我想在可视化中实现一些后期处理效果，为此我需要能够将场景作为纹理进行采样。我已经成功地创建了一个帧缓冲区，并将其颜色纹理渲染为屏幕上的四边形。

我的理解是，与默认相比，Alpha 混合在渲染到屏幕外帧缓冲区时的行为不同。我无法在网上找到任何资源来指示一种无需重大重构即可产生相同结果的方法。我所看到的方法包括手动渲染对象，从后到前按顺序渲染对象，或者将 alpha 值烘焙到发送到帧缓冲区的颜色。我尝试了一种经常建议的替代方案，即使用 glBlendFuncSeparate 更手动地控制事物：

glEnable(GL_BLEND); 
glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE_MINUS_SRC_ALPHA);

但这并没有导致我的结果有任何明显的改善（我也不会期望它，因为这里的数学无法解决我所看到的混合问题）。

到现在，到一些实际的代码上，就够了。我的代码库非常庞大，所以很遗憾我不能分享所有这些，因为有许多专有的绘图例程，但我可以从如何生成帧缓冲区开始：

// Create the framebuffer object
glGenFramebuffers(1, &m_fbo);

// Bind the framebuffer to the current context
glBindFramebuffer(GL_FRAMEBUFFER, m_fbo);

// generate texture to attach as a color attachment to the current frame buffer
m_texColorUnit = 4;
// Set to width and height of window, and leave data uninitialized
glGenTextures(1, &m_texColorBuffer);
glActiveTexture(GL_TEXTURE0 + m_texColorUnit);
glBindTexture(GL_TEXTURE_2D, m_texColorBuffer);
glTexImage2D(GL_TEXTURE_2D, 
    0, 
    GL_RGB8_OES,
    m_navigation->renderContext()->getWidth(),
    m_navigation->renderContext()->getHeight(),
    0, 
    GL_RGB8_OES,
    GL_UNSIGNED_BYTE,
    NULL);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

// attach texture to currently bound framebuffer object
glFramebufferTexture2D(GL_FRAMEBUFFER, 
    GL_COLOR_ATTACHMENT0, 
    GL_TEXTURE_2D, 
    m_texColorBuffer,
    0);

glBindTexture(GL_TEXTURE_2D, 0); //unbind the texture
glActiveTexture(GL_TEXTURE0);    // Reset active texture to default

// Create renderBuffer object for depth and stencil checking
glGenRenderbuffers(1, &m_rbo);
glBindRenderbuffer(GL_RENDERBUFFER, m_rbo); // bind rbo
glRenderbufferStorage(GL_RENDERBUFFER, 
    GL_DEPTH24_STENCIL8_OES, 
    m_navigation->renderContext()->getWidth(),
    m_navigation->renderContext()->getHeight()
); // allocate memory

// Attach rbo to the depth and stencil attachment of the fbo
glFramebufferRenderbuffer(GL_FRAMEBUFFER, 
    GL_DEPTH_STENCIL_OES, 
    GL_RENDERBUFFER,
    m_rbo);

以及大气的着色器：

// vert
#ifndef GL_ES
precision mediump int;
precision highp float;
#endif

attribute vec3 posAttr;

uniform highp mat4 matrix;
uniform highp mat4 modelMatrix;

uniform vec3 v3CameraPos;       // The camera's current position
uniform vec3 v3LightPos;        // The direction vector to the light source
uniform vec3 v3InvWavelength;   // 1 / pow(wavelength, 4) for the red, green, and blue channels
uniform float fCameraHeight;    // The camera's current height
uniform float fCameraHeight2;   // fCameraHeight^2
uniform float fOuterRadius;     // The outer (atmosphere) radius
uniform float fOuterRadius2;    // fOuterRadius^2
uniform float fInnerRadius;     // The inner (planetary) radius
uniform float fInnerRadius2;    // fInnerRadius^2
uniform float fKrESun;          // Kr * ESun
uniform float fKmESun;          // Km * ESun
uniform float fKr4PI;           // Kr * 4 * PI
uniform float fKm4PI;           // Km * 4 * PI
uniform float fScale;           // 1 / (fOuterRadius - fInnerRadius)
uniform float fScaleDepth;      // The scale depth (i.e. the altitude at which the atmosphere's average density is found)
uniform float fScaleOverScaleDepth; // fScale / fScaleDepth

const int nSamples = 5;
const float fSamples = 5.0;

varying vec3 col;
varying vec3 colatten;
varying vec3 v3Direction;
varying vec3 vertexWorld;

float scale(float fCos)
{
    float x = 1.0 - fCos;
    return fScaleDepth * exp(-0.00287 + x*(0.459 + x*(3.83 + x*(-6.80 + x*5.25))));
}

void main(void)
{
    // Get the ray from the camera to the vertex and its length (which is the far point of the ray passing through the atmosphere)

    vec3 v3Pos = posAttr;
    vec3 vertexWorld = posAttr;
    vec3 v3Ray = v3Pos - v3CameraPos;

    float fFar = length(v3Ray);
    v3Ray /= fFar;

    // Calculate the closest intersection of the ray with the outer atmosphere (which is the near point of the ray passing through the atmosphere)
    float B = 2.0 * dot(v3CameraPos, v3Ray);
    float C = fCameraHeight2 - fOuterRadius2;
    float fDet = max(0.0, B*B - 4.0 * C);
    float fNear = 0.5 * (-B - sqrt(fDet));

    // Calculate the ray's starting position, then calculate its scattering offset
    vec3 v3Start = v3CameraPos + v3Ray*fNear;
    fFar -= fNear;
    float fStartAngle = dot(v3Ray, v3Start) / fOuterRadius;
    float fStartDepth = exp(-1.0 / fScaleDepth);
    float fStartOffset = fStartDepth*scale(fStartAngle);

    // Initialize the scattering loop variables
    float fSampleLength = fFar / fSamples;
    float fScaledLength = fSampleLength * fScale;
    vec3 v3SampleRay = v3Ray * fSampleLength;
    vec3 v3SamplePoint = v3Start + v3SampleRay * 0.5;

    // Now loop through the sample rays
    vec3 v3FrontColor = vec3(0.0, 0.0, 0.0);
    for(int i=0; i<nSamples; i++)
    {
        float fHeight = length(v3SamplePoint);
        float fDepth = exp(fScaleOverScaleDepth * (fInnerRadius - fHeight));
        float fLightAngle = dot(v3LightPos, v3SamplePoint) / fHeight;
        float fCameraAngle = dot(v3Ray, v3SamplePoint) / fHeight;
        float fScatter = (fStartOffset + fDepth*(scale(fLightAngle) - scale(fCameraAngle)));
        vec3 v3Attenuate = exp(-fScatter * (v3InvWavelength * fKr4PI + fKm4PI));
        v3FrontColor += v3Attenuate * (fDepth * fScaledLength);
        v3SamplePoint += v3SampleRay;
    }

    // Finally, scale the Mie and Rayleigh colors and set up the varying variables for the pixel shader
    colatten = v3FrontColor * fKmESun;
    col = v3FrontColor * (v3InvWavelength*fKrESun);
    v3Direction = v3CameraPos - v3Pos;
    gl_Position = matrix * modelMatrix * vec4(posAttr,1);
}

// frag
#ifdef GL_ES
precision highp float;
precision mediump int;
#endif

varying vec3 col;
varying vec3 colatten;
varying vec3 v3Direction;
varying vec3 vertexWorld;

uniform vec3 v3LightPos;
uniform float g;
uniform float g2;
uniform float fExposure;

void main (void)
{
    //float fCos = dot(normalize(lPos), normalize(v3Direction));
    float fCos = dot(v3LightPos, v3Direction) / length(v3Direction);
    float fRayleighPhase = 0.75 * (1.0 + fCos*fCos);
    float fMiePhase = 1.5 * ((1.0 - g2) / (2.0 + g2)) * (1.0 + fCos*fCos) / pow(1.0 + g2 - 2.0*g*fCos, 1.5);
    //vec3 result = clamp(col + fMiePhase * colatten, vec3(0,0,0), vec3(1,1,1));
    //gl_FragColor = vec4(result, result.b);
    gl_FragColor.rgb = 1.0 - exp(-fExposure * (fRayleighPhase * col + fMiePhase * colatten));
    //gl_FragColor.a = 1.0;

    gl_FragColor.a = gl_FragColor.b;
}

正如我所说，我的成绩并不出色。第一张图片是我在渲染到屏幕外帧缓冲区时得到的，第二张图片是我直接渲染到屏幕时得到的。关于如何解决这两个问题的任何想法？