How to get Sprite:hitTestPoint to ignore transparent areas of image

chipster123 · December 2011

Is there any way to get the Sprite:hitTestPoint to be aware of the transparent areas of the image so the return value can be 'false' ?
Currently, I can grab Sprites by their transparent areas, which looks kind of bad.

mykyl66 · December 2011

I knew there was something I had meant to mention. I would also like the ability to ignore transparent areas of images.

M

atilim · December 2011

Currently no. I've added this to the requests list. (But per-pixel testing can be tricky because you should store one texture for GPU and one texture for CPU and this means twice as much memory consumption as before)

But there may be some workarounds such as representing the bitmap with a coarse polygon and doing a "point in polygon" test. I can post a point in polygon test code in Lua here.

chipster123 · December 2011

I look foward to it.

atilim · December 2011

hi chipster123,

sorry for the late reply. here is the function:

function pointInPolygon(vertices, x, y)
	local intersectionCount = 0
 
	local x0 = vertices[#vertices].x - x
	local y0 = vertices[#vertices].y - y
 
	for i=1,#vertices do
		local x1 = vertices[i].x - x
		local y1 = vertices[i].y - y
 
		if y0 > 0 and y1 <= 0 and x1 * y0 > y1 * x0 then
			intersectionCount = intersectionCount + 1
		end
 
		if y1 > 0 and y0 <= 0 and x0 * y1 > y0 * x1 then
			intersectionCount = intersectionCount + 1
		end
 
		x0 = x1
		y0 = y1	
	end
 
	return (intersectionCount % 2) == 1
end

You should create the vertices like:

vertices = {
	{x = 50, y = 20},
	{x = 200, y = 200},
	{x = 10, y = 160}
}

Also I recommend PhysicsEditor http://www.physicseditor.de/ to easily create polygons from your images.

Best,

chipster123 · December 2011

Atilim,
Thanks for the info.
I'll try that some time.

Chip

RickT · January 2012

Thanks Atilim, this was very helpful.

tkhnoman · April 2013

Very helpful indeed. And using PhysicsEditor help much.

Code for getting vertices:

 
function getTouchVertices(filename)
	local vertices = {}
	local phydataGet = (loadfile "publishedFromPhysicsEditor.lua")().physicsData(1.0)
 
	for fi,fixture in pairs(phydataGet.data[filename].fixtures) do
 
		vertices[fi] = {}
		if(fixture.shape) then
			for i = 1,#fixture.shape do
				local vnum = math.ceil(i / 2)
				if i % 2 == 1 then 
					vertices[fi][vnum] = {}
					vertices[fi][vnum].x = fixture.shape[i]
				else
					vertices[fi][vnum].y = fixture.shape[i]
				end
			end
		end
	end	
 
 
	return vertices
end
object.vertices = getTouchVertices("something")

And in case of rotating object:

local xo,yo = object:getPosition()
local deltaX,deltaY = xMouse - xo, yMouse - yo
 
local rotatione = -math.atan2(deltaX, deltaY)* 180 / math.pi
local length= math.sqrt(deltaX* deltaX + deltaY * deltaY)
local degree = -math.rad( rotatione - object:getRotation() )
local xToCheck = math.sin(degree) * length 
local yToCheck = math.cos(degree) * length 
 
for i = 1,#object.vertices do
	if pointInPolygon(object.vertices[i], xToCheck,yToCheck) then
		--return true
	end
end

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