OpenGL glReadPixels returns 0

当我呈现到屏幕缓冲区时，一切都很好，但是当从glreadpixels读取framebuffer的像素时，它总是会返回0。

伪代码如下：

将纹理绑定到Framebuffer：

GLuint texture;
glGenTextures(1, &texture);
glBindTexture(GL_TEXTURE_2D, texture);
int width, height;
width = 2;
height = 2;
float texture_data[] = {
    1.0f, 0.0f, 0.0f,   1.0f, 1.0f, 1.0f,
    1.0f, 1.0f, 1.0f,   1.0f, 0.0f, 0.0f
};
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, width, height, 0, GL_RGB, GL_FLOAT, texture_data);
GLuint framebuffer;
glGenFramebuffers(1, &framebuffer); 
glBindFramebuffer(GL_FRAMEBUFFER, framebuffer); 
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texture, 0); 
GLint texture_coord_attribute = glGetAttribLocation(program, "texture_coord");
glEnableVertexAttribArray(texture_coord_attribute);
glVertexAttribPointer(texture_coord_attribute, 2, GL_FLOAT, GL_FALSE,
    sizeof(vertices[0]), (void*)(sizeof(float) * 5));

fragment/vertex着色器：

static const char* vertex_shader_text =
"#version 330n""
"attribute vec2 vPos;n"
"attribute vec2 texture_coord;n"
"varying vec2 Texcoord;n"
"void main()n"
"{n"
"    gl_Position = vec4(vPos, 0.0, 1.0);n"
"    Texcoord = texture_coord;n"
"}n";
static const char* fragment_shader_text =
"#version 330n"
"varying vec2 Texcoord;n"
"uniform sampler2D tex;n"
"void main()n"
"{n"
"    gl_FragColor = texture(tex, Texcoord);n"
"}n";

在主循环中读取像素：

glViewport(0, 0, 720, 480);
glUseProgram(program);
glDrawArrays(GL_TRIANGLES, 0, 6);
GLubyte pixels[3] = {0};
glReadBuffer(GL_COLOR_ATTACHMENT0);
glReadPixels(360, 240, 1, 1, GL_RGB, GL_FLOAT, pixels);
// Any value returns 0 not only 360 and 240
printf("|%f||%f||%f|n", pixels[0], pixels[1], pixels[2]);
glfwSwapBuffers(window);
glfwPollEvents();

这是我遵循的管道。这里怎么了？谢谢。