c++ - Weird behaviour of Z-Buffer -

i'm using deferred shading method render scene, have problem skybox technique due weird behaviour of z-buffer. i've created additional framebuffer , attached 5 textures, 1 of being used depth buffer. first pass geometry pass, fills textures needed data, second pass renders final objects textures , lightning applied main framebuffer, last pass renders skybox. problem skybox being ignored (z-buffer check fails, invisible).

when rendering skybox, map secondary framebuffer gl_read_framebuffer , main framebuffer (screen) gl_draw_framebuffer, depth buffer of secondary framebuffer can used, set depth function gl_lequal. , if change depth function gl_greater (or gl_always) skybox can seen, being rendered on top of objects.

the vertex shader:

#version 330  layout (location = 0) in vec3 position;  uniform mat4 m; uniform mat4 v; uniform mat4 p;  out vec3 texcoord0;  void main() {     vec4 mvp_pos = p * v * m * vec4(position, 1.0);     gl_position = mvp_pos.xyww;     texcoord0 = position; } 

model matrix product of translation of skybox camera coordinates.

gldepthfunction set gl_lequal:

gldepthfunction set gl_greater:

contents of z-buffer:

update 1: i've tried use glblitframebuffer that:

glbindframebuffer(gl_read_framebuffer, m_fbo); //additional framebuffer glbindframebuffer(gl_draw_framebuffer, 0); gldepthmask(gl_true); glblitframebuffer(0, 0, window_width, window_height, 0, 0, window_width, window_height, gl_depth_buffer_bit, gl_nearest); 

but doesn't work. main depth buffer contains array of black pixels.

update 2:

//fbo initialization glgenframebuffers(1, &m_fbo); glbindframebuffer(gl_draw_framebuffer, m_fbo);  // create gbuffer textures glgentextures(array_size_in_elements(m_textures), m_textures); glgentextures(1, &m_depthtexture);  (unsigned int = 0; < array_size_in_elements(m_textures); i++) {     glbindtexture(gl_texture_2d, m_textures[i]);      gltexparameterf(gl_texture_2d, gl_texture_min_filter, gl_nearest);     gltexparameterf(gl_texture_2d, gl_texture_mag_filter, gl_nearest);      glteximage2d(gl_texture_2d, 0, gl_rgb32f, windowwidth, windowheight, 0, gl_rgb, gl_float, null);      glframebuffertexture2d(gl_draw_framebuffer, gl_color_attachment0 + i, gl_texture_2d, m_textures[i], 0);     }  // depth buffer glbindtexture(gl_texture_2d, m_depthtexture); glteximage2d(gl_texture_2d, 0, gl_depth_component32f, windowwidth, windowheight, 0, gl_depth_component, gl_float,         null); glframebuffertexture2d(gl_draw_framebuffer, gl_depth_attachment, gl_texture_2d, m_depthtexture, 0);  glenum drawbuffers[] = { gl_color_attachment0, gl_color_attachment1, gl_color_attachment2, gl_color_attachment3 }; gldrawbuffers(array_size_in_elements(drawbuffers), drawbuffers);  glenum status = glcheckframebufferstatus(gl_framebuffer);  if (status != gl_framebuffer_complete) {     fprintf(stderr, "fb error, status: 0x%x\n", status);     return false; } 

skybox buffers:

glfloat cube_vertices[] = {         // front         -1.0, -1.0, 1.0,         1.0, -1.0, 1.0,         1.0, 1.0, 1.0,         -1.0, 1.0, 1.0,         //         -1.0, -1.0, -1.0,         1.0, -1.0, -1.0,         1.0, 1.0, -1.0,         -1.0, 1.0, -1.0,     };      glushort cube_elements[] = {         // front         0, 1, 2,         2, 3, 0,         // top         3, 2, 6,         6, 7, 3,         //         7, 6, 5,         5, 4, 7,         // bottom         4, 5, 1,         1, 0, 4,         // left         4, 0, 3,         3, 7, 4,         // right         1, 5, 6,         6, 2, 1,     }; 

when rendering skybox, map secondary framebuffer gl_read_framebuffer , main framebuffer (screen) gl_draw_framebuffer, depth buffer of secondary framebuffer can used,

that not how things work. gl_read_framebuffer not relevant depth test. used source opeations glreadpixels or glblitframebuffer. kind of rendering, gl_draw_framebuffer relevant. includes depth test.

unfortuanly, can't mix user-defined render targets render buffers or textures window-system provided buffers, can't render final screen while using specific depth test. either have additional render pass fbo using custom depth buffer, , blit result screen, or can blit contents of custom depth buffer window-system provided 1 , render that.