==== Perspective ====

by Bitbreaker/Oxyron/Nuance

Best is to calculate the perspective during multiply with the rotation matrix. As soon as you get the value for Z, lookup a corresponding factor in a table and multply the results for X and Y with that factor:

<code>
        ... matrix multipplication for Z ...
        tay
        lda z_fact,y
        sta z1
        eor #$ff
        sta z2
        
        ... matrix multiplication for X ...
        
        tay
        ;multiply with z_fact
        lda (z1),y
        sec
        sbc (z2),y
        sta final_x
        
        ... matrix multiplication for Z ...
        
        tay
        ;multiply with z_fact
        lda (z1),y
        sec
        sbc (z2),y
        sta final_y
</code>

The factor-table you could generate like the following:
<code>
    ...
    d = 280.0; z0 = 5.0;
    for (i = 0; i < 0x100; i++) {
        z = i;
        //make things signed
        if(z > 127) z = z - 256;
        q = round(d/(z0-z/64.0));
        //take care that values are sane
        if(q > 127) q = 127;
        if(q < -127) q = -127;

        if(q < 0) q = 256 + q;

        result[i] = q;
    }
    ...
</code>