Some elements are flashing

[orsinium]

ROM:

https://github.com/kripod/chip8-roms/blob/master/games/Pong%20%5BPaul%20Vervalin%2C%201990%5D.ch8

When playing in other emulators (like https://emuchip.com/), no elements are flashing. However, in rsc8, the pads constantly disappear and appear again.

[orsinium]

So, what happens is that chip-8 apps all are naturally flashing since these computers didn't have frame buffers and every operation was updating a pixel on the screen directly. Some other emulators solve it by having a 4-8 frames fading trail: instead of turning off the pixel immediately, it "fades" instead. This makes elements like pads in Pong "dim" a bit instead of disappearing completely.

In case of rsc8, a simple solution could be to solve it on the TUI side:

const TRAILS: u8 = 4;
let mut fading_screen = [0u8; 32 * 64];
// ...

for (i, set) in chip8.screen.iter().enumerate() {
    if *set {
        fading_screen[i] = TRAILS;
    } else {
        fading_screen[i] = fading_screen[i].saturating_sub(1);
    }
}

And then when rendering, 0 will be black (off), 1-3 will be shades of gray, and 4 will be black.

However, there is still one more issue left. rsc8 runs 8 (TICK_PER_FRAME) opcodes before rendering each frame and the frame (Chip8.screen) contains only the latest screen state. This means that in some games (like Cave.ch8), it might happen that the flashing element is always off on 8, it will be always invisible. So, a solution that I found working the best is to actually change Chip8::screen to track the fading. Then if a pixel flashes and disappears between screen updates, we'll still track it as slightly faded.

Before every update, fade the non-on pixels:

for px in &mut self.screen {
    if *px != TRAILS {
        *px = px.saturating_sub(1)
    }
}

Then 00E0 fades all pixels:

for px in &mut self.screen {
    *px = px.saturating_sub(1);
}

And Dxyn :

if sprite_pixel && screen_pixel == TRAILS {
    self.reg_v[0xF] = 1;
}
self.screen[screen_pixel_index] =
    match (screen_pixel == TRAILS, sprite_pixel) {
        (false, false) => screen_pixel,
        (true, false) | (false, true) => TRAILS,
        (true, true) => TRAILS - 1,
    }

[jerryshell]

Hi @orsinium, thank you so much for the detailed report – it's really helpful.
I'm quite busy with work at the moment, so I may not be able to address these issues right away.
I'll get back to them as soon as I can. Really appreciate your contribution!

[orsinium]

Take your time :) We're both doing it for fun.

It's a very good CHIP-8 implementation, great job!