OpenGL：从FrameBuffer中读取像素的四舍五入取整为255（0xFF）

Question

我试图通过将vao id打包到RGBA并使用它渲染到屏幕缓冲区，然后尝试使用缓冲对象。

/* create a framebuffer object */ 
glGenFramebuffers(1, &fbo);  
/* attach the texture and the render buffer to the frame buffer */ 
glBindFramebuffer(GL_FRAMEBUFFER, fbo); 

/* generate a texture id */ 
glGenTextures(1, &tex); 
/* bind the texture */ 
glBindTexture(GL_TEXTURE_2D, tex); 
/* create the texture in the GPU */ 
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, WINDOW_SIZE_X, WINDOW_SIZE_Y 
    , 0, GL_RGBA, GL_UNSIGNED_BYTE, nullptr); 

/* set texture parameters */ 
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE); 
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE); 
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST); 
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST); 

/* unbind the texture */ 
glBindTexture(GL_TEXTURE_2D, 0); 

/* create a renderbuffer object for the depth buffer */ 
glGenRenderbuffers(1, &rbo); 
/* bind the texture */ 
glBindRenderbuffer(GL_RENDERBUFFER, rbo); 
/* create the render buffer in the GPU */ 
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT 
    , WINDOW_SIZE_X, WINDOW_SIZE_Y); 

/* unbind the render buffer */ 
glBindRenderbuffer(GL_RENDERBUFFER, 0); 


/* attach the texture and the render buffer to the frame buffer */ 
glFramebufferTexture(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, tex, 0); 
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT 
    , GL_RENDERBUFFER, rbo); 

// check the frame buffer 
if (glCheckFramebufferStatus( 
    GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE) { 
     std::cout << "Framebuffer status not complete" << '\n'; 
} 
/* handle an error : frame buffer incomplete */ 
/* return to the default frame buffer */ 
glBindFramebuffer(GL_FRAMEBUFFER, 0); 

我还产生一个像素缓冲器对象读取：

我被创建纹理和z缓冲器渲染对象然后将它们附加到帧缓冲区对象作为这样渲染到屏幕外的缓冲器从帧缓冲区后渲染完成：

/* generate the pixel buffer object */ 
glGenBuffers(1,&pbo);  
glBindBuffer(GL_PIXEL_PACK_BUFFER, pbo); 
glBufferData(GL_PIXEL_PACK_BUFFER, WINDOW_SIZE_X * WINDOW_SIZE_Y * 4, nullptr, GL_STREAM_READ); 
/* to avoid weird behaviour the first frame the data is loaded */ 
glReadPixels(0, 0, WINDOW_SIZE_X, WINDOW_SIZE_Y, GL_BGRA, GL_UNSIGNED_BYTE, 0);  

然后在我的渲染循环我绑定，并呈现到屏幕外的帧缓冲：

GLubyte red, green, blue, alpha; 

/* bind the frame buffer */ 
glBindFramebuffer(GL_FRAMEBUFFER, fbo); 

/* clear the frame buffer */ 
glClearColor(0,0,0,0); 
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); 

/* select the shader program */ 
glUseProgram(pickProgram); 
/* set the object color */ 

/*alpha = house.vaoId & 0xFF; 
blue = (house.vaoId >> 8) & 0xFF; 
green = (house.vaoId >> 16) & 0xFF; 
red = (house.vaoId >> 24) & 0xFF; */ 

GLuint objectId = 5; 
alpha = objectId & 0xFF; 
blue = (objectId >> 8) & 0xFF; 
green = (objectId >> 16) & 0xFF; 
red = (objectId >> 24) & 0xFF; 

//Upload the packed RGBA values to the shader 
glUniform4f(baseColorUniformLocation, red, green ,blue, alpha);  

    //prepare to draw the object 
pvm = projectionMatrix*viewMatrix*house.modelMatrix; 
glUniformMatrix4fv(offScreenMatrixUniformLocation, 1, GL_FALSE, glm::value_ptr(pvm)); 

/* draw the object*/ 
glBindVertexArray(house.getVaoId()); 
glDrawRangeElements(GL_TRIANGLES,0,42,42,GL_UNSIGNED_SHORT,NULL); 
glBindVertexArray(0); 

//check that our framebuffer is ok 
if(glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE) { 
std::cout << "Framebuffer Error" << '\n'; 
} 

GLuint temp; 
    //get the object id from the read pixels 
temp = get_object_id(); <--- this function is explained later 


/* return to the default frame buffer */ 
glBindFramebuffer(GL_FRAMEBUFFER, 0); 

呈现到离屏缓冲器的片段着色器是非常简单的：

#version 420 
uniform vec4 BaseColor; 
layout(location=0) out vec4 fragColor; 

void main() 
{ 
fragColor = BaseColor; 
} 

这是离屏描绘期间调用以从帧缓冲器中提取封装的RGBA功能：

GLuint Engine::get_object_id() 
{ 
static int frame_event = 0; 
GLuint object_id; 
int x, y; 
GLuint red, green, blue, alpha, pixel_index; 
//GLuint read_pbo, map_pbo; 
GLubyte* ptr; 

/* read one pixel buffer */ 
glBindBuffer(GL_PIXEL_PACK_BUFFER, pbo_a); 
/* map the other pixel buffer */ 
    ///////////////////// NOTE :5th argument, BGRA or RGBA, doesn't make a difference right? 
glReadPixels(0, 0, WINDOW_SIZE_X, WINDOW_SIZE_Y, GL_BGRA, GL_UNSIGNED_BYTE, 0); 
ptr = (GLubyte*)glMapBuffer(GL_PIXEL_PACK_BUFFER, GL_READ_WRITE); 
/* get the mouse coordinates */ 
/* OpenGL has the {0,0} at the down-left corner of the screen */ 
glfwGetMousePos(&x, &y); 
y = WINDOW_SIZE_Y - y; 
object_id = -1; 
if (x >= 0 && x < WINDOW_SIZE_X && y >= 0 && y < WINDOW_SIZE_Y){ 
////////////////////////////////////////// 
    //I have to admit I don't understand what he does here 
/////////////////////////////////////////// 
    pixel_index = (x + y * WINDOW_SIZE_X) * 4; 
    blue = ptr[pixel_index]; 
    green = ptr[pixel_index + 1]; 
    red = ptr[pixel_index + 2]; 
    alpha = ptr[pixel_index + 3]; 

object_id = alpha +(red << 24) + (green << 16) + (blue << 8); 
} 
glUnmapBuffer(GL_PIXEL_PACK_BUFFER); 
glBindBuffer(GL_PIXEL_PACK_BUFFER, 0); 
return object_id; 
} 

问题是，这应该从屏幕外帧缓冲区读取像素并给我vao_id的代码的最后一点有以下奇怪的行为：

如果任何打包到RGBA（通过着色器发送）中的4个字节是除0之外的任何字节，而在另一端，该字节以0xFF出现。

所以，如果我送

00000000 00000000 00000000 00000001 

我会得到

00000000 00000000 00000000 11111111 

或者我叫

00000000 00010000 00000000 00000000 

我会得到

00000000 11111111 00000000 00000000 

...当我用get_object_id（）读取像素时。

如果我绑定纹理并将其渲染为正常FrameBuffer上的四边形，那么传递给离屏渲染的颜色会在四边形上正确显示。但是由get_object_id（）读取的像素使每个发送的字节四舍五入为255（0xFF）。所以我的猜测是最终功能有问题。

Answer 1

片段着色器输出写入帧缓冲区时重映射为[0，255]的[0,1]范围内的值。 glUniform4f接受浮点值，你需要把你的ID为[0,1]值，而不是[0,255]值：

glUniform4f(baseColorUniformLocation, red/255.0f, green/255.0f...);