项目需求，要把矩形的头像（或任意图片）剪切成圆形的。然后在网上找到一些方法，并做了简单修改。这里做下记录。

cc.exports.display = display or {}

function display.createMaskedSprite(srcFile,maskFile)
    local src = display.newSprite(srcFile)
    local mask = display.newSprite(maskFile)

    local size_src = src:getContentSize()
    local size_mask = mask:getContentSize()

    local canva = cc.RenderTexture:create(size_src.width,size_src.height,cc.TEXTURE2_D_PIXEL_FORMAT_RGB_A8888)
    
    local ratiow = size_src.width / size_mask.width
    local ratioh = size_src.height / size_mask.height
    mask:setScaleX(ratiow)
    mask:setScaleY(ratioh)

    mask:setPosition(size_src.width / 2,size_src.height / 2)
    src:setPosition(size_src.width / 2,size_src.height / 2)

    local blendfunc_mask = cc.blendFunc(gl.ONE,gl.ZERO)
    mask:setBlendFunc(blendfunc_mask)
    local blendfunc_src = cc.blendFunc(gl.DST_ALPHA,gl.ZERO)
    src:setBlendFunc(blendfunc_src)

    canva:begin()
    mask:visit()
    src:visit()
    canva:endToLua()

    local masked_sprite = cc.Sprite:createWithTexture(canva:getSprite():getTexture())
    masked_sprite:setFlippedY(true)
    return masked_sprite
end

function display.createCircleSprite(srcFile,maskFile)
	local src = display.newSprite(srcFile)
    local mask = display.newSprite(maskFile)

    local size_src = src:getContentSize()
    local size_mask = mask:getContentSize()

    local canva = cc.RenderTexture:create(size_mask.width,size_mask.height,cc.TEXTURE2_D_PIXEL_FORMAT_RGB_A8888)
    
    local ratiow = size_mask.width / size_src.width
    local ratioh = size_mask.height / size_src.height
    src:setScaleX(ratiow)
    src:setScaleY(ratioh)

    mask:setPosition(size_mask.width / 2,size_mask.height / 2)
    src:setPosition(size_mask.width / 2,size_mask.height / 2)

    local blendfunc_mask = cc.blendFunc(gl.ONE,gl.ZERO)
    src:setBlendFunc(blendfunc_src)

    canva:begin()
    mask:visit()
    src:visit()
    canva:endToLua()

    local masked_sprite = cc.Sprite:createWithTexture(canva:getSprite():getTexture())
    masked_sprite:setFlippedY(true)
    return masked_sprite
end

local sp = display.createMaskedSprite("head.png","mask.png")