Moving sprite leaves traces

simme · May 25, 2022, 6:27am

Hey,

I've got two sprites:

the wall that uses draw commands to just draw an outline
the ball that is also just a draw command to fill a circle

The ball has a vector and in its update method I just move it by this vector. What I can't figure out is why the ball is leaving traces like this? I've tried filling the rect of the frame sprite to clear it out, but that doesn't seem to work. I feel like I'm missing something about the sprite/drawing system?

Here's a gist of the entire code (general pointers welcome, although this is just a super quick mockup)

gist.github.com

https://gist.github.com/simme/fd7235517a733f3717f8de4d154d5565

main.lua

import "CoreLibs/object"
import "CoreLibs/graphics"
import "CoreLibs/sprites"
import "CoreLibs/timer"

local gfx <const> = playdate.graphics
local geometry  <const> = playdate.geometry

local levelWidth = 360
local levelHeight = 204

This file has been truncated. show original

Thanks

dave · May 25, 2022, 1:44pm

The (x,y,w,h) parameters passed to the sprite callback give the dirty rect (in sprite-local coordinates with top-left origin) that needs redrawn. That trail happens because the World sprite is drawing its border inside the update rects as the ball moves instead of in the full sprite rect. You could change the code to always draw the border at full size, or you can use the dirty rect params to avoid drawing the border if it's not needed and save a bit of CPU:

function World:draw(x, y, w, h)
  gfx.setColor(gfx.kColorWhite)
  gfx.fillRect(x, y, w, h)
  gfx.setColor(gfx.kColorBlack)

  local selfw, selfh = self:getSize()

  if x <= 2 or x+w >= selfw-4 or y <= 2 or y+h >= selfh-4 then
    gfx.setStrokeLocation(gfx.kStrokeInside)
    gfx.drawRoundRect(0, 0, selfw, selfh, 2)
    gfx.drawRect(2, 2, selfw - 4, selfh - 4)
  end
end

simme · May 25, 2022, 2:26pm

Ah, of course. That makes perfect sense. And is clearly stated in the docs... sorry!