Does anybody have any hints on how to use the Virtual Boy’s affine mode to get a Mode-7 like project as used in Mode 0. I’ve managed to create a background and upload it, and get my affine parameters set up, but I’ve run into a wall.
From what I’ve gathered, affine mode (ignore parallax) does two things. For a given row, it permits you to choose a pixel in the original background image (as loaded in BG memory) which becomes the new starting pixel for the row as drawn on the screen. In other words, affine mode first translates the BG image before the VIP commences drawing. Next, affine mode will draw adjacent pixels for the row by consulting an increment value for skipping pixels (fractional pixels allowed- hence fixed point) in the current row of original background image. This is used to implement a scale.
Translation and scaling happen in this order, and I’ve found it difficult to take a background of parallel vertical lines and apply a perspective transform such that the parallel vertical lines converge to a single point in the distance. The affine transform needs to be varied on a line-by-line basis, and I have not found the correct combination of scale and increment to accomplish this. Has anyone had success with a perspective transform in the past? Well, since F-Zero was PORTED to VB using homebrew, I’d guess so :P.
The following code is the best I’ve been able to do so far, and… it’s not great. The lines are too curved, and the view is not centered (adding the y-axis is going to be fun XD):
#include "external/libgccvb/libgccvb.h"
const char char_line[] = {
0xC0, 0x03, 0xC0, 0x03, 0xC0, 0x03, 0xC0, 0x03, \
0xC0, 0x03, 0xC0, 0x03, 0xC0, 0x03, 0xC0, 0x03 \
};
int main()
{
int row_cnt;
//Initial setup
vbDisplayOn();
setmem((void*)BGMap(0), 0x00, 0x2000); /* Just use the zeroth char for all tiles */
for(row_cnt = 0; row_cnt < 224; row_cnt++)
{
short x_skip, x_offset;
char * copy_base = (char *) (BGMap(1) + (16*row_cnt)); /* Calculate the base once per loop. */
/* Y-coordinate should increment by row compared to original BG (1-to-1) */
(* (copy_base + 4)) = (row_cnt << 3); /* 13.3 */
(* (copy_base + 5)) = ((row_cnt & 0x7F) >> 3); /* 13.3 */ /* WHY is the top bit a sign bit?! */
/* X-inc per pixel- 7.9 */ /* However, change the increment value per row! */
x_skip = ((255 - row_cnt) << 2);
x_offset = row_cnt << 3;
(* (short *) (copy_base + 0)) = x_offset;
(* (short *) (copy_base + 6)) = x_skip;
(* (short *) (copy_base + 8)) = 0; /* Y-inc */
}
copymem((void*)CharSeg0, (void*)char_line, 16); /* Only need to use 1 char! */
//Setup worlds
WA[31].head = (WRLD_ON | WRLD_AFFINE | WRLD_OVR);
WA[31].gx = 0;
WA[31].gp = 0; //No parallax for now.
WA[31].gy = 75; // Use 2/3s of the screen.
WA[31].w = 384;
WA[31].h = 149;
WA[31].ovr = 0;
WA[31].param = 0x01000; /* Affine Parameters begins after BG 0 (0x1000*2+0x20000) */
WA[30].head = WRLD_END;
//Set brightness registers
vbDisplayShow();
//Main loop (Empty because we're done but don't want to reach the end)
for(;;);
return 0;
}
Attachments:
Something I didn’t realize about F-Zero until about 2 minutes ago- the camera is slightly angled downward. This permits the perspective projection to take up more than 50% of the screen.
Something I forgot until tonight: If my camera is looking straight down the negative z-axis (or just looking directly down any axis, for that matter), and I place two parallel lines within my camera’s point of view, those parallel lines will converge toward the middle of the screen, such as the attached image of a long rectangle with a camera looking down the z-axis. That means, unless I tilt my camera, a mathematically-sound perspective effect can only take up to 50% of the screen (likely less, since 50% is the limiting case).
In F-Zero, the perspective projection (Mode 7 effects), takes up 75% of the screen!
Contrast to Star Fox, from Mode 2 horizon boundary maxes out at looks like 50% of the screen (i.e. the bottom of the camera’s field-of-view IS crashing into the solid ground :P)!
Therefore, my perspective transform as is (horizon is at 66% of the screen, camera looking straight down negative z-axis), is not possible XD. I’ll rework it and see if I can come up with a better one. Tilting my camera downwards slightly was not something I really had in mind (I’d rather the camera strictly follow the backside of my main 3d object, but that might not be a good idea either).
