How to constrain emission particles? So, I'm attempting to create random lit windows for buildings in a scene, and someone suggested using a particle system and use the textures as emission planes. So far I can create the lights, but getting them where I want them, and making them the size of the faces is another story. Here's what is happening:



Above I tried setting up a mesh that consisted of faces for each window (green), and a vertex group to go with it (weight 1.0). I couldn't get the particles to stay in the faces as shown. I also moved the particle system to the building, and used a copy of the specular map texture to control it, while using the influence of size/length (1.0) for the particles. Still, the result was like the above. 

It feels like I'm missing something small, but not obvious. I'm wanting the emission to come from the separate faces, or from the black portions of the texture, as well as each being the size of each. Can anyone shed some light on this issue? 

Thanks in advance!
Animation below.





In the image below note the settings Particles/Face set to 1. Face Jittered Random. These setting respect the faces created in the geometry.  This the preferred setting for more varied geometry. The image has an extra windows on the right side to show how these settings respect faces.  Plane subdivision is a requirement.