depth-write pre-pass to fight overdraw. inspired by Doom3

gistfile1.txt

static void draw_frame (double elapsed) {
	glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
	if (do_pre_pass) { // depth-writing pre-pass
		glUseProgram (dshader_programme);
		for (int i = 0; i < NUM_MONKEYS; i++) {
			float x = 0.0f;
			float y = 0.0f;
			float z = sinf ((float)i) * 100.0f - 100.0f;
			mat4 M = translate_mat4 (vec3_from_3f (x, y, z));
			mat4 PVM = mult_mat4_mat4 (PV, M);
			glUniformMatrix4fv (dsp_PVM_loc, 1, GL_FALSE, PVM.m);
			glBindVertexArray (mesh.vao);
			glDrawArrays (GL_TRIANGLES, 0, mesh.pc);
		}
		glDepthFunc (GL_LEQUAL); // because self is gonna be equal duh!
		glDepthMask (GL_FALSE); // disable depth writing - already done
	}
	{ // normal render pass with no depth rendering
		glUseProgram (shader_programme);
		for (int i = 0; i < NUM_MONKEYS; i++) {
			float x = 0.0f;
			float y = 0.0f;
			float z = sinf ((float)i) * 100.0f - 100.0f;
			mat4 M = translate_mat4 (vec3_from_3f (x, y, z));
			mat4 PVM = mult_mat4_mat4 (PV, M);
			glUniformMatrix4fv (sp_PVM_loc, 1, GL_FALSE, PVM.m);
			glBindVertexArray (mesh.vao);
			glDrawArrays (GL_TRIANGLES, 0, mesh.pc);
		}
		glDepthFunc (GL_LESS);
		glDepthMask (GL_TRUE); // disable depth writing - already done
	}
	glfwSwapBuffers (g_gl.win);
}