Question

我试图通过将vao id打包到RGBA并使用它渲染到屏幕外缓冲区来实现对象拾取，然后尝试用缓冲区对象读取。

我通过创建纹理和z缓冲区RenderBuffer对象渲染到屏幕外缓冲区，然后将它们附加到FrameBuffer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);

我还生成一个像素缓冲区对象，在渲染完成后从FrameBuffer中读取：

    /* 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中，我将其绑定并渲染到屏幕外的FrameBuffer：

    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);

渲染到屏幕外缓冲区的片段着色器非常简单：

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

void main() 
{ 
 fragColor = BaseColor; 
}

这是在屏幕外渲染期间调用的函数，用于从帧缓冲区中提取打包的RGBA：

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; 
}

问题是这个应该从屏幕外的Framebuffer中读取像素并给我vao_id的最后一段代码有以下奇怪的行为：

如果打包到RGBA中的4个字节中的任何一个（通过着色器发送）是0以外的任何字节，则该字节在另一端出现为0xFF。

所以，如果我发送

00000000 00000000 00000000 00000001

我会得到

00000000 00000000 00000000 11111111

或者如果我发送

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到我的普通FrameBuffer上的四边形，那么我传递到屏幕外渲染的颜色在四边形上正确显示。但是get_object_id（）读取的像素会使每个发送的字节向上舍入到255（0xFF）。所以我猜这个最终功能有问题。

Answer 1

片段着色器输出[0,1]范围内的值，当写入帧缓冲区时会重新映射到[0,255]。 glUniform4f接受浮点值，您需要将您的ID作为[0,1]值而不是[0,255]值发送：

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

OpenGL：读取FrameBuffer中的像素，用于拾取舍入到255（0xFF）

1 个答案: