Problem about "previous render" always black after each frame of anim rendering

[helensmith1]

Previous render - this parameter determines what should be done with the previous image in the virtual frame buffer when rendering starts. Note that this parameter has no effect on the final result of the rendering; it is implemented simply as a convenient way to distinguish between parts from the current frame being rendered, and parts left over from the previous rendering.

 

The possible values are:

• Unchanged - no changes will be made - the virtual frame buffer remains the same;
  
• Cross - every second pixel of the image will be set to black;
  
• Fields - every other line of the image will be set to black;
  
• Darken - the colors in the image will be darkened.
  
• Blue - the previous image is tinted in blue.
  

 

But my problem with this setting is no matter what I set it up in Vray panel, it always empty (black) after each frame (when the frame rendering got done and go to the next). It's supposed to keep the previous render in max frame buffer, isn't it.

 

Is there any other reason lead to this issue? I'm sure there's one 'coz I have some previous project render without problem with this setting but recently, the animation rendering always turn out black after each frame. I guess it has something to do with pre-calculating setting (pre-calc map saving/lightcache/IRR precalc..) but never really figure it out. So hope anyone has the answer or idea for this, thanks