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I'm making an interior scene in which all light comes from a window with a curtain. This is an image that shows the effect I'm trying to achieve:
The light source is a plane with an emissive material outside the room. Without the curtain, the scene looks nice. However, I can't find a way to make the scene illuminated and have the curtain look good at the same time.
Here's a scene before the curtain. The only light is an emission plane outside and over the window.
(clik on the images to enlarge)
If we add a translucent curtain we get a lot less light illuminating the room.
Adding transparency will allow more light into the room, but will make the curtain "see-trough".
Controlling the transparency so that is not visible to the camera will make the light come into the room while keeping the translucency of the curtain's material:
By changing the ray depth values you can control how much light comes into the room:
Alternatively
You can just turn the whole curtain into an emission object unseen by the camera:
In production environment it is sometimes necessary to cheat light source in the scene, as there is no way that we can bounce light around a room in the exact way the real world does it. In a 3d environment there will be a limit to the amount of time a light bounces around, in the real world ... pretty much infinite.
You will often read about artist creating light sources that aren't suppose to be there in order for 3d scenes to look right. That's where the artistic license come into play. In fact in real life photography, professional camera operator often use reflectors and strobe lights to create extra lighting information to illuminate a rather boring room.
If you do not have enough light in the scene place and area light source around or beside your camera and tilt it around. Like a photographer with a strobe light and play around with it, bounce the light source off a wall or the ceiling, or even a mirror. Using the semi real-time feedback in the cycle render viewport , you should be able to achieve great looking images in no time.
The Second option is to composite your image. Render your image in a higher 32 bit image format like the exr format. Using this format you can adjust the exposure of the image... using different exposure for different part of the room would be one of the tricks artist use to produce compelling interior rendering.
