b2dDrawQuadDepth

Like b2dDrawQuad, but every corner carries its own depth value (z1..z4, each 0 = near .. 1 = far) instead of the single depth set by b2dSetDepth. The GPU interpolates the depth across the face per pixel, so faces that INTERPENETRATE or are steeply tilted resolve correctly against each other - the case a single flat per-quad depth cannot handle. fill = 1 fills it, 0 draws the outline. Honours the current colour and alpha. Use b2dDrawQuad (+ b2dSetDepth) for flat 2D quads; use this for true 3D-in-2D faces where per-pixel depth matters.

IMPORTANT: supply PERSPECTIVE-CORRECT depths. The corner values are interpolated linearly across the screen (like any GPU vertex attribute), so a planar face needs projective depth - a value of the form a + b/z (the same shape as clip-space Z), NOT linear view-Z. Feeding linear view-Z makes a planar quad's depth crease along its triangle split (a visible "kink" where faces meet). A near/far mapping that works: dep = (FAR * (z - NEAR)) / (z * (FAR - NEAR)).