mm/src/code/z_viszbuf.c

/**
 * @file z_viszbuf.c
 *
 * This file implements a full-screen framebuffer effect for visualizing the z-buffer (AKA depth buffer), using either
 * cycling RGBA or a single fading color.
 *
 * This is done by reading the z-buffer as if it were a color image, the format of which is specified by the selected
 * vis type:
 *  - VIS_ZBUF_TYPE_IA   : Produces a monotonic fade from primColor to envColor as depth increases.
 *  - VIS_ZBUF_TYPE_RGBA : Produces vibrant almost-periodic-looking bands.
 *
 * In both cases this occurs because of the format the depth information takes: it is 18-bit, and is a nonnegative
 * floating-point number with
 *     bbb      mmmmmmmmmmm dd|dd
 *     exponent mantissa    dz value (only first 16 bits visible to CPU, the least significant 2 bits of dz are ignored)
 *
 * Reading z-buffer as IA16:
 *     bbbmmmmm mmmmmmdd
 *     iiiiiiii aaaaaaaa
 *
 * Since floating-point numbers of this format have the same ordering as their binary/hex representation, increasing
 * the depth also increases the intensity in the IA16 representation and hence the interpolation parameter used to
 * combine primColor and envColor. The alpha is ignored by the RenderMode.
 *
 * Reading z-buffer as RGBA16:
 *     bbbmm mmmmm mmmmd d
 *     rrrrr ggggg bbbbb a
 *
 * The red increases monotonically with the depth. The significant visible oscillation is the green component, because
 * it rolls over every time the second-most-significant bit of the mantissa increments. The blue component oscillates
 * too rapidly to be particularly visible (it rolls over when the 7th-most-significant bit increments). The alpha is
 * again ignored by the RenderMode.
 */
#include "z64vis.h"
#include "global.h"
#include "sys_cfb.h"
#include "stdbool.h"

// Height of the fragments the z-buffer is split into.
// It is the maximum amount of lines such that all rgba16 SCREEN_WIDTH-long lines fit into TMEM.
#define VISZBUF_ZBUFFRAG_HEIGHT (TMEM_SIZE / (gCfbWidth * G_IM_SIZ_16b_BYTES))

/**
 * Initialise to IA type with white and black as default colors.
 */
void VisZBuf_Init(VisZBuf* this) {
    this->vis.type = VIS_ZBUF_TYPE_IA;
    this->vis.scissorType = VIS_NO_SETSCISSOR;
    this->vis.primColor.r = 255;
    this->vis.primColor.g = 255;
    this->vis.primColor.b = 255;
    this->vis.primColor.a = 255;
    this->vis.envColor.r = 0;
    this->vis.envColor.g = 0;
    this->vis.envColor.b = 0;
    this->vis.envColor.a = 255;
}

void VisZBuf_Destroy(VisZBuf* this) {
}

void VisZBuf_Draw(VisZBuf* this, Gfx** gfxP, void* zbuffer) {
    Gfx* gfx = *gfxP;
    s32 height = VISZBUF_ZBUFFRAG_HEIGHT;
    s32 y;
    s32 fmt = (this->vis.type == VIS_ZBUF_TYPE_IA) ? G_IM_FMT_IA : G_IM_FMT_RGBA;

    if (zbuffer == NULL) {
        return;
    }

    gDPPipeSync(gfx++);
    if (this->vis.scissorType == VIS_SETSCISSOR) {
        gSPDisplayList(gfx++, D_0E000000.setScissor);
    }

    // No palette so can use all of TMEM.
    // G_RM_OPA_SURF discards all information previously in the pixel, and the current alpha, leaving only the color
    // from this filter.
    gDPSetOtherMode(gfx++,
                    G_AD_DISABLE | G_CD_MAGICSQ | G_CK_NONE | G_TC_FILT | G_TF_POINT | G_TT_NONE | G_TL_TILE |
                        G_TD_CLAMP | G_TP_NONE | G_CYC_1CYCLE | G_PM_NPRIMITIVE,
                    G_AC_NONE | G_ZS_PRIM | G_RM_OPA_SURF | G_RM_OPA_SURF2);

    // LERP between primColor and envColor in 1-cycle mode using the z-buffer value.
    gDPSetCombineLERP(gfx++, PRIMITIVE, ENVIRONMENT, TEXEL0, ENVIRONMENT, PRIMITIVE, ENVIRONMENT, TEXEL0, ENVIRONMENT,
                      PRIMITIVE, ENVIRONMENT, TEXEL0, ENVIRONMENT, PRIMITIVE, ENVIRONMENT, TEXEL0, ENVIRONMENT);
    gDPSetColor(gfx++, G_SETPRIMCOLOR, this->vis.primColor.rgba);
    gDPSetColor(gfx++, G_SETENVCOLOR, this->vis.envColor.rgba);

    for (y = 0; y < (gCfbHeight - height) + 1; y += height) {
        // Load a few lines of the z-buffer, as many as can fit in TMEM at once.
        gDPLoadTextureTile(gfx++, zbuffer, fmt, G_IM_SIZ_16b, gCfbWidth, 0, 0, y, gCfbWidth - 1, (y + height) - 1, 0,
                           G_TX_NOMIRROR | G_TX_CLAMP, G_TX_NOMIRROR | G_TX_CLAMP, G_TX_NOMASK, G_TX_NOMASK, G_TX_NOLOD,
                           G_TX_NOLOD);

        // Overwrite them with the calculated colors.
        gSPTextureRectangle(gfx++, 0, y << 2, gCfbWidth << 2, (y + height) << 2, G_TX_RENDERTILE, 0, y << 5, 1 << 10,
                            1 << 10);
    }

    gDPPipeSync(gfx++);
    *gfxP = gfx;
}