OnRenderImage with multi camera

Hey,that's a problem when i use OnRenderImage.

There are 2 cameras in my scene.“A” camera only render the layer “Terrain” with depth -1 and clear with skybox.

And “B” Camera render others with depth 0 and clear nothing. Well,all the things wrok correctly.

Than I use OnRenderImage in "A" to get a decal effect only on the terrain,but on the screen I CAN'T see the decal......The shader is same as shadowmap.

I'm sure the decal shader is right because I can see the decal on the ground when I disable the camera B.

So......Why OnRenderImage can't work correctly when there is another Camera with larger depth?