QB64-PE/internal/c/mtri3.cpp

mtri3_usegrad3:;

if (final == 1) {
    if (no_edge_overlap)
        y2 = y2 - 1;
}

// not on screen?
if (y1 >= dheight) {
    return;
}
if (y2 < 0) {
    if (final)
        return;
    // jump to y2's position
    // note; original point locations are referenced because they are unmodified
    // & represent the true distance of the run
    y = y2 - y1;
    p1 = g1->p1;
    p2 = g1->p2;
    d = g1->y2 - g1->y1;
    if (d) {
        i64 = p2->tx - p1->tx;
        g1->tx += i64 * y / d;
        i64 = p2->ty - p1->ty;
        g1->ty += i64 * y / d;
        i64 = p2->x - p1->x;
        g1->x += i64 * y / d;
        p1 = g2->p1;
        p2 = g2->p2;
    }
    d = g2->y2 - g2->y1;
    if (d) {
        i64 = p2->tx - p1->tx;
        g2->tx += i64 * y / d;
        i64 = p2->ty - p1->ty;
        g2->ty += i64 * y / d;
        i64 = p2->x - p1->x;
        g2->x += i64 * y / d;
    }
    goto mtri3_final;
}

// clip top
if (y1 < 0) {
    // note; original point locations are referenced because they are unmodified
    // & represent the true distance of the run
    y = -y1;
    p1 = g1->p1;
    p2 = g1->p2;
    d = g1->y2 - g1->y1;
    if (d) {
        i64 = p2->tx - p1->tx;
        g1->tx += i64 * y / d;
        i64 = p2->ty - p1->ty;
        g1->ty += i64 * y / d;
        i64 = p2->x - p1->x;
        g1->x += i64 * y / d;
        p1 = g2->p1;
        p2 = g2->p2;
    }
    d = g2->y2 - g2->y1;
    if (d) {
        i64 = p2->tx - p1->tx;
        g2->tx += i64 * y / d;
        i64 = p2->ty - p1->ty;
        g2->ty += i64 * y / d;
        i64 = p2->x - p1->x;
        g2->x += i64 * y / d;
    }
    y1 = 0;
}

if (y2 >= dheight) { // clip bottom
    y2 = dheight - 1;
}

// move indexed variable values into direct variables for faster referencing
// within 2nd bottleneck
g1x = g1->x;
g2x = g2->x;
g1tx = g1->tx;
g2tx = g2->tx;
g1ty = g1->ty;
g2ty = g2->ty;
g1xi = g1->xi;
g2xi = g2->xi;
g1txi = g1->txi;
g2txi = g2->txi;
g1tyi = g1->tyi;
g2tyi = g2->tyi;

// 2nd bottleneck
for (y = y1; y <= y2; y++) {

    if (g1x < 0)
        x1 = (g1x - 65535) / 65536;
    else
        x1 = g1x / 65536; // int-style rounding of fixed-point value
    if (g2x < 0)
        x2 = (g2x - 65535) / 65536;
    else
        x2 = g2x / 65536;

    if (x1 >= dwidth | x2 < 0)
        goto mtri3_donerow; // crop if(entirely offscreen

    tx = g1tx;
    ty = g1ty;

    // calculate gradients if they might be required
    if (x1 != x2) {
        d = g2x - g1x;
        i64 = g2tx - g1tx;
        txi = (i64 << 16) / d;
        i64 = g2ty - g1ty;
        tyi = (i64 << 16) / d;
    } else {
        txi = 0;
        tyi = 0;
    }

    // calculate pixel offsets from ideals
    loff = ((g1x & 65535) - 32768); // note; works for positive & negative
                                    // values
    roff = ((g2x & 65535) - 32768);

    if (roff < 0) { // not enough of rhs pixel exists to use
        if (x2 < dwidth & no_edge_overlap == 0) { // onscreen check
            // draw rhs pixel as is
            //<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
            *(dst_offset + (y * dwidth + x2)) =
                src_offset[(g2ty >> 16) * swidth + (g2tx >> 16)];
            //<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
        }
        // move left one position
        x2--;
        if (x1 > x2 | x2 < 0)
            goto mtri3_donerow; // no more to do
    } else {
        if (no_edge_overlap) {
            x2 = x2 - 1;
            if (x1 > x2 | x2 < 0)
                goto mtri3_donerow; // no more to do
        }
    }

    if (loff > 0) {
        // draw lhs pixel as is
        if (x1 >= 0) {
            //<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
            *(dst_offset + (y * dwidth + x1)) =
                src_offset[(ty >> 16) * swidth + (tx >> 16)];
            //<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
        }
        // skip to next x location, effectively reducing steps by 1
        x1++;
        if (x1 > x2)
            goto mtri3_donerow;
        loff = -(65536 - loff); // adjust alignment to jump to next ideal offset
    }

    // align to loff
    i64 = -loff;
    tx += (i64 * txi) / 65536;
    ty += (i64 * tyi) / 65536;

    if (x1 < 0) { // clip left
        d = g2x - g1x;
        i64 = g2tx - g1tx;
        tx += ((i64 << 16) * -x1) / d;
        i64 = g2ty - g1ty;
        ty += ((i64 << 16) * -x1) / d;
        if (x1 < 0)
            x1 = 0;
    }

    if (x2 >= dwidth) {
        x2 = dwidth - 1; // clip right
    }

    //<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
    pixel_offset = dst_offset + (y * dwidth + x1);
    //<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

    // bottleneck
    for (x = x1; x <= x2; x++) {

        //<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
        *(pixel_offset++) = src_offset[(ty >> 16) * swidth + (tx >> 16)];
        //<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

        tx += txi;
        ty += tyi;
    }

mtri3_donerow:;

    if (y != y2) {
        g1x += g1xi;
        g1tx += g1txi;
        g1ty += g1tyi;
        g2x += g2xi;
        g2tx += g2txi;
        g2ty += g2tyi;
    }
}

if (final == 0) {

    // update indexed variable values with direct variable values which have
    // changed & may be required
    g1->x = g1x;
    g2->x = g2x;
    g1->tx = g1tx;
    g2->tx = g2tx;
    g1->ty = g1ty;
    g2->ty = g2ty;

mtri3_final:;
    if (y2 < dheight - 1) { // no point continuing if(offscreen!
        if (g1->y2 < g2->y2)
            g1 = g3;
        else
            g2 = g3;

        // avoid doing the same row twice
        y1 = g3->y1 + 1;
        y2 = g3->y2;
        g1->x += g1->xi;
        g1->tx += g1->txi;
        g1->ty += g1->tyi;
        g2->x += g2->xi;
        g2->tx += g2->txi;
        g2->ty += g2->tyi;

        final = 1;
        goto mtri3_usegrad3;
    }
}

return;