As the screenshot below shows, when I pack the sprite's texture as ATF format, at the same time check 'additive' blending option at the slot, then the additive object just doesn't show up and only darken artifact shows behind the character...

I'm using the runtime of starling version.
please help me with this problem.....

Normal PNG texture

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Using ATF (both character and background object)

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Detailed explanation about ATF :
http://www.adobe.com/devnet/flashruntimes/articles/introducing-compressed-textures.html
http://www.adobe.com/devnet/flashruntimes/articles/atf-users-guide.html
http://jacksondunstan.com/articles/2040

ATF texture can save a lot of GPU memory and loading time, very helpful.

I found that those darken artifacts appear when:

• All textures used nearby are ATF texture (including none-spine object)
• some part of spine object uses 'Additive' blending
• that additive blending object also has 'color' animation (mainly fade-out effect using alpha)

I also found that using 'SCREEN' blending instead of 'ADD' can prevent darken artifact.
I edited code here :
https://github.com/EsotericSoftware/spine-runtimes/blob/master/spine-starling/spine-starling/src/spine/starling/SkeletonSprite.as#L110

I'm not sure if this is related with ATF texture's none premultiplied alpha feature.
The code below is from starling.display.BlendModes.as, which was some fix for similar ATF artifact bug (from starling version 1.3)

private static var sBlendFactors:Array = [ 
            // no premultiplied alpha
            { 
                "none"     : [ Context3DBlendFactor.ONE, Context3DBlendFactor.ZERO ],
                "normal"   : [ Context3DBlendFactor.SOURCE_ALPHA, Context3DBlendFactor.ONE_MINUS_SOURCE_ALPHA ],
                "add"      : [ Context3DBlendFactor.SOURCE_ALPHA, Context3DBlendFactor.DESTINATION_ALPHA ],
                "multiply" : [ Context3DBlendFactor.DESTINATION_COLOR, Context3DBlendFactor.ONE_MINUS_SOURCE_ALPHA ],
                "screen"   : [ Context3DBlendFactor.SOURCE_ALPHA, Context3DBlendFactor.ONE ],
                "erase"    : [ Context3DBlendFactor.ZERO, Context3DBlendFactor.ONE_MINUS_SOURCE_ALPHA ]
            },
            // premultiplied alpha
            { 
                "none"     : [ Context3DBlendFactor.ONE, Context3DBlendFactor.ZERO ],
                "normal"   : [ Context3DBlendFactor.ONE, Context3DBlendFactor.ONE_MINUS_SOURCE_ALPHA ],
                "add"      : [ Context3DBlendFactor.ONE, Context3DBlendFactor.ONE ],
                "multiply" : [ Context3DBlendFactor.DESTINATION_COLOR, Context3DBlendFactor.ONE_MINUS_SOURCE_ALPHA ],
                "screen"   : [ Context3DBlendFactor.ONE, Context3DBlendFactor.ONE_MINUS_SOURCE_COLOR ],
                "erase"    : [ Context3DBlendFactor.ZERO, Context3DBlendFactor.ONE_MINUS_SOURCE_ALPHA ]
            }
        ];

Isn't there any solution for 'ADD' blending instead of 'SCREEN' ? . . .