OpenGL strange depth test behavior

Question

I have a simple OpenGL program that renders terrain with a texture. It exhibits some strange behaviors (see video) that look similar to depth test issues. However, I have already enabled depth test. Where else can I look to solve this problem?

https://www.youtube.com/watch?v=WHTFa3tMSyQ

This is the relevant setup code

glEnable(GL_BLEND);
glEnable(GL_DEPTH_TEST);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

... VBO and texture setup ..

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

projectionMatrix = glm::frustum(-2.0, 2.0, -2.0, 2.0, 1.0, 10000.0);

... rendering stuff ...

glFlush();
swapEGLBuffers();

@DMGregory According to that link, that is enabled by default. I have not turned it off. — user8908459
– user8908459, Commented Sep 7, 2019 at 2:44
Can you show us how you define the output you're rendering into, including its depth buffer? — DMGregory
– DMGregory ♦, Commented Sep 7, 2019 at 2:55
It seems to me like you don't have a depth buffer requested during surface creation. — Ocelot
– Ocelot, Commented Sep 7, 2019 at 3:03
If you've solved your problem, then you're in a better position to share the code that solved it. It's 100% OK to answer your own questions here. :) — DMGregory
– DMGregory ♦, Commented Sep 7, 2019 at 11:11

user8908459 · Accepted Answer · 2019-09-09 17:55:58Z

The problem was that I did not request a depth buffer in my EGL config. The solution was to add EGL_DEPTH_SIZE to my requested EGL configuration:

So this:

EGLint attrib_list[]= { EGL_RED_SIZE,        8,
                        EGL_GREEN_SIZE,      8,
                        EGL_BLUE_SIZE,       8,
                        EGL_SURFACE_TYPE,    EGL_WINDOW_BIT,
                        EGL_RENDERABLE_TYPE, 0,
                        EGL_NONE};

Became this:

EGLint attrib_list[]= { EGL_RED_SIZE,        8,
                        EGL_GREEN_SIZE,      8,
                        EGL_BLUE_SIZE,       8,
                        EGL_SURFACE_TYPE,    EGL_WINDOW_BIT,
                        EGL_RENDERABLE_TYPE, 0,
                        EGL_DEPTH_SIZE, 24,
                        EGL_NONE};

This answer on stackoverflow helped: https://stackoverflow.com/questions/13349714/messed-up-opengl-depth-buffer

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