//init func

//----------------------
	//FRONT
	//----------------------
	
	glGenFramebuffers(1, &m_FrameBufferFront);
	glBindFramebuffer(GL_FRAMEBUFFER, m_FrameBufferFront);
	
	glGenTextures(1, &m_RenderedTextureFront);
	glBindTexture(GL_TEXTURE_2D, m_RenderedTextureFront);
	glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT, 800, 600, 0, GL_DEPTH_COMPONENT, GL_FLOAT, 0);

	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);

	glFramebufferTexture(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, m_RenderedTextureFront, 0);

	//----------------------
	//BACK
	//----------------------
	
	glGenFramebuffers(1, &m_FrameBufferBack);
	glBindFramebuffer(GL_FRAMEBUFFER, m_FrameBufferBack);
	
	glGenTextures(1, &m_RenderedTextureBack);
	glBindTexture(GL_TEXTURE_2D, m_RenderedTextureBack);
	glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT, 800, 600, 0, GL_DEPTH_COMPONENT, GL_FLOAT, 0);

	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST); 
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);

	glFramebufferTexture(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, m_RenderedTextureBack, 0);

	glDrawBuffer(GL_NONE);

	if (glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE)
	{
		return;
	}

void CMeshManager::Draw_MeshManager()
{
	glEnable(GL_CULL_FACE);
	glEnable(GL_DEPTH_TEST);
	
	//------------------------------
	//FRONT PASS 1
	//------------------------------
	glBindFramebuffer(GL_FRAMEBUFFER, m_FrameBufferFront);
	
	glViewport(0,0,800,600);

	glClearColor(0.0f, 0.0f, 0.0f, 0.0f );
	glClearDepth(1.0f);
    glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );

	glUseProgram(m_ProgramHandleDepth);
	glBindVertexArray(m_FsCube);
	
	GLuint MatMVP = glGetUniformLocation(m_ProgramHandleDepth, "MVP");
	if( MatMVP >= 0 )
    {
        glUniformMatrix4fv(MatMVP, 1, GL_FALSE, (const GLfloat*)&m_MatVP);
    }

	GLuint ZFar = glGetUniformLocation(m_ProgramHandleDepth, "ZFar");
	if (ZFar >= 0)
	{
		glUniform1f(ZFar, m_ZFar);
	}

	glCullFace(GL_BACK);
	
	glDrawElements(GL_TRIANGLES, 36,GL_UNSIGNED_INT, 0);

	glBindVertexArray(0);
	glUseProgram(0);
	
	//------------------------------
	//BACK PASS 2
	//------------------------------
	glBindFramebuffer(GL_FRAMEBUFFER, m_FrameBufferBack);

	glViewport(0,0,800,600);

	glClearColor(0.0f, 0.0f, 0.0f, 0.0f );
	glClearDepth(1.0f);
    glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );

	glUseProgram(m_ProgramHandleDepth);
	glBindVertexArray(m_FsCube);
	
	MatMVP = glGetUniformLocation(m_ProgramHandleDepth, "MVP");
	if( MatMVP >= 0 )
    {
        glUniformMatrix4fv(MatMVP, 1, GL_FALSE, (const GLfloat*)&m_MatVP);
    }

	ZFar = glGetUniformLocation(m_ProgramHandleDepth, "ZFar");
	if (ZFar >= 0)
	{
		glUniform1f(ZFar, m_ZFar);
	}

	glCullFace(GL_FRONT);
	glDrawElements(GL_TRIANGLES,36,GL_UNSIGNED_INT, 0);

	glBindVertexArray(0);
	glUseProgram(0);
	
	//---------------
	//FOG PASS 3
	//---------------
	
	glBindFramebuffer(GL_FRAMEBUFFER, 0);
	glViewport(0,0,800,600);

	glEnable (GL_BLEND);
	glBlendFunc (GL_ONE, GL_ONE);
	
	glDisable (GL_DEPTH_TEST);
	glCullFace(GL_BACK);

	glClearColor(0.0f, 0.125f, 0.3f, 1.0f);
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
	
	glUseProgram(m_ProgramHandleFog);
	glBindVertexArray(m_FsQuad);

	glActiveTexture(GL_TEXTURE0);
	glBindTexture( GL_TEXTURE_2D, m_RenderedTextureFront );
	
	GLuint Tex = glGetUniformLocation(m_ProgramHandleFog, "ourTexture_Front");
	if( Tex >= 0 )
    {
        glUniform1i(Tex, 0);
    }
	
	glActiveTexture(GL_TEXTURE1);
	glBindTexture( GL_TEXTURE_2D, m_RenderedTextureBack );
	
	Tex = glGetUniformLocation(m_ProgramHandleFog, "ourTexture_Back");
	if( Tex >= 0 )
    {
        glUniform1i(Tex, 1);
    }
	
	glDrawArrays(GL_TRIANGLES, 0, 6);
	
	glBindVertexArray(0);
	glUseProgram(0);
    
}

//depth.vert
#version 330

layout (location = 0) in vec3 aPos;

out float TexDepth;

uniform mat4 MVP;
uniform float ZFar;

void main()
{
	vec4 PositionT = MVP * vec4(aPos,1.0f);

	gl_Position = PositionT;

	TexDepth=PositionT.w/ZFar;
}

//depth.frag
#version 330

out float FragColor;

in float TexDepth;

void main()
{
    gl_FragDepth = TexDepth;
}

//fog.frag
#version 330

out vec4 FragColor;
  
in vec2 tex;

uniform sampler2D ourTexture_Front;
uniform sampler2D ourTexture_Back;

uniform vec3 FogColor = {0.5f,0.5f,0.5f}; 

float FogFactor = 12.0f;
 
void main()
{

	float front = texture(ourTexture_Front, tex).r;
	float back = texture(ourTexture_Back, tex).r;

	if(front == 1.0f)
		front = 1.0f - front;
	
	if(back == 1.0f)
		back = 1.0f - back;

	float k = (back-front) * FogFactor;

	FragColor = vec4(FogColor*k, 1.0f);

}

I make volumetric fog. I draw a cube on the stage, in the first depth texture I record the depth of the front polygons of the cube, in the second depth texture I record the depth of the rear polygons of the cube. Then I set both textures in the shader, subtract the difference in depth from both textures - this is the intensity of the fog. I display the pixel with intensity on the screen.

The problem is that the code does not work correctly.

This code draws like this:

https://ibb.co/KXhksC1

It should be like this:

https://ibb.co/HnvWVsR

Who knows what's wrong?