A detailed explanation about Z buffering and Z values will appear in later sections. Here we will focus on the procedures you need to go through in order to make use of the Z buffer. To make active use of Z buffering in an N64 program, you must follow the three procedures below to prepare for use of the Z buffer.

1. Set the geometry mode to G_ZBUFFER
2. Set the rendering mode to a mode that uses the Z buffer
3. Clear the Z buffer at the start of every frame (before rendering the primitive)

No special explanation is required about the geometry mode. You simply place the mode constant you want to set in the gSPSetGeometryMode command.

gSPSetGeometryMode(glistp++, G_ZBUFFER | G_SHADE | G_SHADING_SMOOTH |
                             G_CULL_BACK);

For the rendering mode, gfx7.c uses G_RM_AA_ZB_OPA_SURF. Modes with "ZB" in their name have a macro definition that specifies the Z buffer enable flag Z_CMP. If you want to read more about the rendering modes, see the Programming Manual, Chapter 15.7, "Blender Modes and Assumptions."

  /* Opaque - anti-aliasing - Z buffering */
  gDPSetRenderMode(glistp++, G_RM_AA_ZB_OPA_SURF, G_RM_AA_ZB_OPA_SURF2);

To register and clear the Z buffer you need to put together a number of RDP commands, but it is not a specialized operation. To explain the procedure we will use the example of the ClearBackground function from gfx7.c. The code is shown below:

  gDPPipeSync(glistp++);
  gDPSetCycleType(glistp++, G_CYC_FILL);
  /* Set the Z buffer  */
  gDPSetDepthImage(glistp++, nuGfxZBuffer);
  /* Specify the subject of coloring (Z buffer) */
  gDPSetColorImage(glistp++, G_IM_FMT_RGBA, G_IM_SIZ_16b, SCREEN_WD,
                             nuGfxZBuffer);
  /* Specify the color (background Z value, maximum Z value) */
  gDPSetFillColor(glistp++, (GPACK_ZDZ(G_MAXFBZ, 0) << 16 |
                             GPACK_ZDZ(G_MAXFBZ, 0)));
  /* Clear the Z buffer */
  gDPFillRectangle(glistp++, 0, 0, SCREEN_WD - 1, SCREEN_HT - 1);

In this code, the gDPSetDepthImage command is used to tell the RDP the address of the Z buffer. In this case, the NuSystem global variable nuGfxZBuffer is given as the Z buffer address. Registering the Z buffer is completed with the call of this one command. Of course, the pointer passed to this command must be a segment address. As mentioned before, in this tutorial physical address 0 is mapped to segment address 0, so you can pass the virtual address as is.

The next necessary step is to clear (initialize) the Z buffer. You do this the same way as when you clear the framebuffer by using a combination of the gDPSetColorImage command and gDPSetFillColor command. However, in the case of the Z buffer, the value you use when clearing is not the value for a color, but the value for the maximum Z value. In our example from gfx7.c, the Z buffer is cleared using the macro constant G_MAXFBZ, which indicates the maximum Z value, and the DeltaZ value of 0. When clearing (or filling) the Z buffer, typically the GPACK_ZDZ macro defined in <gbi.h> is used to pack the fill value. We will bring up the topic of the DeltaZ value in the next section.