Not everyone has the capabilities to render huge models with a ton of lights. I am one of those people where more times than not I have to manually add light to my illustrations because I don't have the computing power. At first, this method can seem tedious. But, when you think about the time that it takes to insert lights into a model and then the time that it takes to render so many lights, knowing how to do this in Photoshop can be a great resource to have as a backup. There is a typical workflow I use that consistently yields clean, realistic results without much effort. I am using the urban snow scene that I posted a few months back as the base case for this tutorial. The model was large, and there were way too many windows to try to render. The steps I used to Photoshop the light are outlined below.
1. SAVE THE REFLECTIONS - For scenes like the one above, I rendered the glass of the surrounding buildings with a strong reflection. This allows me to copy the reflections to their own layer and use them later on as an overlay. To copy them to their own layer, select the "polygonal tool", then select the glass of the windows you intend to light. Right-click on the selection and choose "Layer via Copy".
2. PAINT IN THE BASE LIGHT - Next, I created a new layer called "glass light". Again, I selected the glass part of the window. To speed things up, you can ctrl+click the reflections layer in the previous step instead of using the polygonal tool to reselect all of the glass. With the glass selected, choose the "Brush" tool, select a soft yellow paint color, select a "Soft Round" brush, and lower the opacity of the brush to around 15%. Begin painting color with most of the light at the bottom of the window fading away as you move towards the top of the window.
3. HIGHLIGHTS - This is one of those steps that is easy to forget or to treat as not important. In reality, it is the most crucial step in creating successful lighting. Without it, the light looks flat. Begin by creating a new layer and setting the blend mode to "Overlay". Select only the surfaces facing the window that would be hit by the light coming through the glass. A good way to determine this is by looking for surfaces that are 90 degrees to the glass. With the selections made, choose the "Brush" tool, select a soft yellow paint, select a "Soft Round" brush, and set the opacity to around 35%. Begin painting in the highlights. If the highlights seem weak, you can amplify the results by duplicating the highlight layer.
4. SOFT LIGHT GLOW - Another subtle, but important step. Create a new layer and choose an off white paint color. Choose the "Paint" tool, select a "Soft Round" brush, and set the opacity really low to something like 12% to 15%. Begin painting around the window edges being careful not to overdo it.
5. BRING BACK THE REFLECTIONS - The last step is to retrieve the reflections layer created in the first step. Bring that layer to the top of the layers pallet, and set the blend mode to "Screen". This will amplify the reflections since they were most likely diluted from the painting in the previous steps.