Birth Texture substitute?

[ravihara]

tyFlow's 'Birth Texture' alternative is to do a regular birth and use Position Object's texture density parameter to control where particles are born. Having it animate over time is as simple as birthing more particles over time.

The problem is, when I animate the texture from complete dark to areas of white patches growing over time, it does't show up any particles. It works other way around where dark patches growing from ligh background. I understand its because particles are already sent to the next event by all black state but how to solve this problem?