As a workaround, you could put a tiny offset to one of the RGB channel values, so the Constant Color never outputs (0,0,0,0), or add some logic to check for zero output and turn off the sprite accordingly. I think you can check for null images using JavaScript, and output a boolean that will turn the sprite or billboard on/off.
You can also just plug the image into the enable input of the Sprite -- that's what the "suggested" workaround was for zero-length strings into the Image From String patch's incorrect output (it retains the previous non-zero-length string's dimensions, but is otherwise blank). That's functional, but also fairly lame in my opinion :)
I must admit, I CAN see the logic of an image that will never be seen being disabled. I mean, in a sense, there's no point using resources trying to render an image that is 100% transparent, and will therefore never be visible. Maybe I'm missing the point though.
If alpha is 0, then any value for R, G, and B will be "unseen" so the logic of disabling an invisible image in the name of performance / should/ misfunction on all alpha=0 images (this consistency doesn't happen in practice because of how the ConstantColor generator outputs images). Because of that inconsistency, it's clear it's not an attempted optimization gone awry. Also, optimizing shouldn't affect output, so if it did anything it should do like the billboard and output nothing, not a solidly colored box :)
-- [ christopher wright ] [email protected] http://kineme.net/
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