My render image has a transparent background and when i save it as PNG RGBA every thing is OK. But when i want to save as JPEG format which doesn't support alpha transparency, save it as black backgroud at transparent areas. How to save a render image as JPEG with white background instead of default black background at transparent areas? Just like when i save the render image as PNG RGBA format and open it in GIMP and save as JPEG format which change transparent background to white. Is anyway in blender without using Nodes?
No, but it's quite simple to do with nodes..
In the compositor (press ⎈ Ctrl← Left arrow), enable Use Nodes in the header and add an alpha over node (⇧ ShiftA> Color):
The color in the top slot is the color which will be used instead of transparency.
Now transparency will be automatically filled in with white (or whatever is in the top socket) when the scene is rendered.
You can change the background color by modifying the environment by going to the World tab in the Properties pane. In cycles, change the environment surface color. Turn off everything except camera in in the Ray Visibility panel to prevent the environment from affecting your scene. If you're using Blender Internal renderer, then instead of setting ray visibility, you'll need to set the alpha chanel to display the sky color. The downside of all of this is that you lose the ability to control lighting using the sky map. If that's a problem, the using the compositor to add a backdrop to your renders is the way to go-and this kind of task makes for a good introduction to the compositor.