Because it is an unbiased renderer, Indigo will not tell you when it thinks it is finished. It is up to you to decide when you think the render is clear enough for your purposes. Most renders will give their first image within 30 seconds, but it may take 3/4 hours to give a clear image. Large images (greater than 5 megapixels) may need to be left overnight to become clear.
This is akin to the graininess you get when you take a photo of a dark room with an exposure that is too short. The problem is that not enough light has been simulated to create an accurate representation of the scene. Leave it to render for longer to produce a clear image.
These are called 'fireflies'. They occur when a ray of light randomly reflects into a bright light source, usually the sun. These bright dots also appear in some long exposure digital photography.
There are several settings you can change to reduce fireflies:
* Remove unnecessary glass between the sun and your camera
* Ensure supersampling is set to 3 or 4.
* Set your Rendering mode to Bidirectional with MLT.
* Make sure none of your materials have a color of 1.0, 1.0, 1.0 - all materials should have maximum color values of 0.8, 0.8, 0.8 (no paints in the real world reflect 100% of light)
* Remove the dots using a high pass noise filter in photoshop
When rendering an image, the image is rendered at the specified resolution multiplied by the supersample factor, then reduced for anti-aliasing purposes. The supersampling buffer is also in high dynamic range, so if you are rendering an image of 4000x2000 with a supersampling factor of 4, it will use 384 megabytes of RAM for this buffer.
If you are using a material that has a medium (a specular/glass material), the normals (direction of the mesh faces) must be facing outwards. Also, make sure it is a closed mesh.