skid marks

[Bernd]

Hi,
I'm working on a topdown racing game (GTA like).
I want to draw skid marks of my cars. What would be a fast way to do this?

I tried sf::Shapes in a list, which obviously doesn't work for very long ;-)
Then I tried to have a initially transparent image (same size as the screen) and then use setpixel to draw the skid marks. But even with only one pixel drawn per frame, there is a huge framerate drop.

[Spidyy]

Try this :
http://www.sfml-dev.org/wiki/en/sources/stroke

[Bernd]

Nice project (though a bit overkill for me), but I want to draw the skid marks permanently on the screen.

[Laurent]

I tried sf::Shapes in a list, which obviously doesn't work for very long ;-)

Why? How many marks do you have?

[Bernd]

Everytime a car slides (in a curve for example), I want to draw permanent skidmarks for every tire that slips.

[Laurent]

Ok, but you didn't exactly answer my questions:
- why do you say that sf::Shape doesn't work? did you try it?
- how many marks maximum (approximately) can you have? tens? thousands? millions? all visible on screen?

[Bernd]

I tried shapes. I added a shape to a list every frame (when the car slides), and then drew the hole list. The framerate droppes at a couple hundred shapes.

I don't use a 'camera', so the hole map is always visible, and so are the skidmarks, which probably could easily reach 10000. That's why I wanted to use pixels.

[Laurent]

Ok, understood.

To use an image efficiently, you need to work on your own array of pixels (a std::vector<sf::Uint8> for example, or even a sf::Uint8[] if size is fixed), work on it with direct access, and update the sf::Image in one call with LoadFromPixels. This should be fast enough. SetPixel is not efficient at all, it is made for convenient access to pixels, not for real-time update.

[Bernd]

Thanks Laurent, will try that.

[Bernd]

I got it working now, the code is not complete yet (and contains errors), but the performance is bad :-(

initialization

Code: [Select]

Color c(0,0,0,0);
imgSkids = new Image(SCREEN_WIDTH, SCREEN_HEIGHT, c);
sprtSkids = Sprite(*imgSkids);
int dim = imgSkids->GetWidth() * imgSkids->GetHeight() * 4;
skidmarks = new Uint8[dim];
for (int i = 0; i < dim; i++)
i % 4 != 0 ? skidmarks[i] = 0 : skidmarks[i] = 255;

called once per frame

Code: [Select]


sprtCarPos = sprtCar.GetPosition();
Uint8 *pixel = &skidmarks[(int)((sprtCarPos.x + (sprtCarPos.y * imgSkids->GetWidth())) * 4)];

if (ABS(lateralforce_f.y) > ABS(MAX_GRIP))
{
*pixel = 0;
*(pixel+1) = 0;
*(pixel+2) = 0;
*(pixel+3) = 255;
imgSkids->LoadFromPixels(imgSkids->GetWidth(), imgSkids->GetHeight(), skidmarks);
w.Draw(sprtSkids);
}