python - How do I sort with both depth and y-axis in OpenGL?

Continuing my misadventures in pyOpenGL, I've refactored the whole thing to use 4 buffers:

1. tile vertices - all drawn at the start, probably never modified


2. tile texture co-ords - not modified often (but enables support for animated tiles later)


3. sprite vertices - modified often as sprites move around the map


4. sprite texture co-ords - modified often as sprites animate

My draw loop binds each buffer, and calls drawArrays() twice. I have enabled depth testing, and clear the depth buffer on each draw:

glutInitDisplayMode(GLUT_DOUBLE|GLUT_RGBA|GLUT_DEPTH)

...
glDepthFunc(GL_LESS)
glEnable(GL_DEPTH_TEST)
...
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT)

For the moment, I've hard-coded the z coordinate for each of the things I'm drawing, just to be sure they're definitely in the right order - grass tiles are at 0.9, the tree (4 tiles) is set to -1.0 and the sprites are at 0.0. This is what I get:

In a previous attempt to build this engine without openGL, I was using Tkinter canvas images, which ended up very slow. To get the draw order right, I had to sort all the objects by their y-coordinate plus their z-coordinate, because it is all on a 2D plane. I want something similar, here. I don't understand why the grass tiles cause this problem, when they should be drawn right at the back. GL_DEPTH_TEST is definitely enabled - if I disable it by commenting out the glEnable() call, the sprites are drawn on top of everything (including on top of the trees). I want an illusion of depth by being able to walk behind high-sticking-up objects.

---

I've continued hacking away at this, and I've gotten a little further by modifying my shader:

fragment_shader = shaders.compileShader("""
    uniform sampler2D u_image;

    varying vec2 v_texCoords;

    void main() 
    {
        vec4 tex = texture2D(u_image, v_texCoords);
        if(tex.a < 1.0)

          discard;
        gl_FragColor = tex; //texture2D(u_image, v_texCoords);
    }
""", GL_FRAGMENT_SHADER)

By checking for transparent pixels and throwing them away, I get a lot closer to my goal, however it is still not quite right, and I do not know how to fix it.

The wizard is at z:-0.5, and the tree trunk is at z:-0.2; it is doing what I'm telling it to do, just not what I want it to do. If I put them on the same z-level, the trunk is drawn over the wizard when he stands in front of the tree, and correctly when he stands behind it:

I don't just want depth sorting, I also want to sort on the y-axis. How can I do this in OpenGL?