GL_DEPTH_TEST not work in glsl shader - opengl

Question

Recently, I am working with glsl and fbo in a OpenGL framwork. Here are the details:
Glfw and glew are used to build the OpenGL environment.
I render to a OpenGL window,and also to a fbo (Framebuffer object).
Several render buffers are bind to the fbo, for difference render data, pixels' color, pixels' triangle and vertex id, etc.
A sphere is assigned its texture by "Projected texture" method.
I set glEnable(GL_DEPTH_TEST); before the render loop.
Before each frame I run glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
Create the FBO:
glGenRenderbuffers(1,&m_color_buffer);
glBindRenderbuffer(GL_RENDERBUFFER,m_color_buffer);
glRenderbufferStorage(GL_RENDERBUFFER, GL_RGBA, WindowWidth, WindowHeight);
//////generate other render buffers////////////
glGenFramebuffers(1, &m_fbo);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER,m_fbo);
glFramebufferRenderbuffer(GL_DRAW_FRAMEBUFFER,
GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, m_color_buffer);
//////bind other render buffers////////////
//set the render buffers
GLenum drawBuffers[] = { GL_NONE, GL_COLOR_ATTACHMENT0,
GL_COLOR_ATTACHMENT1, GL_COLOR_ATTACHMENT2, GL_COLOR_ATTACHMENT3 };
glDrawBuffers(5, drawBuffers);
In the fragment shader:
layout( location = 1 ) out vec4 FragColor;
//////////other output variables ///////////////////
void main() {
vec4 projTexColor = vec4(0.0);
if( frag.ProjTexCoord.z > 0.0 )
{
projTexColor = textureProj( ProjectorTex, frag.ProjTexCoord );
}
FragColor = projTexColor;
/////////write other data/////////////
}
RenderBuffer to file:
glBindFramebuffer(GL_READ_FRAMEBUFFER, m_fbo);
glReadBuffer(GL_COLOR_ATTACHMENT0);
GLubyte *m_data = (GLubyte*)malloc(4 * m_width * m_height);
glReadPixels(0, 0, m_width, m_height, GL_RGBA, GL_UNSIGNED_BYTE, m_data);
TGAIO::write(m_data,m_width,m_height,"e:/1.tga");
Here is result:
Render to OpenGL window:
When render to fbo:
It seems that the back part of the sphere appears. And also the front part become fragmented.
So, what's happened ? I do not declare the output for GL_NONE(OpenGL default render buffer) in fragment shader, how does it get the right data ? It seems that OpenGL is smarter than me, at least. LOL.