Today, I'm going to introduce you my method of faking HDRI images for reflections. Have you ever say "if this nice image could be a HDRI map.".At the end of this tutorial, you will be able to make fake HDRI from every picture you have and result will be quite similar to real HDRI reflections.
My method is based on understanding how RGB level(it is located in material editor, in output rollout of every map) affects reflection.If you have for example white, self illuminated object which RGB level is set to 10, this object will appear even in 10% reflection full white. If this object would have set RGB level to 1, it will appear in 10% reflection deep gray.
So we understand how RGB level affects reflections, we can start in explaining my method. First make a mix map. Then assign a map, from which you want to make fake HDRI, into the first slot. Copy it to the second slot as well. Choose copy, not instance! Change the RGB level of second map to something like 10. And now it will come the most interesting part. You will have to make a map in Photoshop that will blend this two images together. Start with bringing the original image into ps, then desaturate it and play with brightness/contrast. In some cases, you will have to blur some part of the image as well. There is no exact procedure to do this map. Just start as I said a practice. As for a example, I used well known kitchen.hdr converted to jpg. Look at the following images - there is map that I have used and also a mask made from this map.
Finally, you can assign this mix map into environment slot and fine tune the mask map or play with RGB level amount of the second map. This method can be also used when you are rendering a interior - make a panoramic image in the middle of the interior, use this method to make a fake HDRI from it, and then use it for reflective things inside your interior. At the end, I want to show you a comparison of my fake HDRI and real HDRI. The first image is done with my method(kitchen.jpg), and the second one is real HDRI(kitchen.hrd). You have to admit that there is just the little difference between these two images.