Optical Effects: Specular Bloom
By Neil Blevins
Created On: Sept 3rd 2000
Updated On:
Apr 18th 2022

Specular Bloom (also called Diffuse Glow, Bloom, Glow, Ghosts, Flare, Glares) is what we call it when the bright objects in your scene glow. This happens in real photography for a number of different reasons, bright areas can be so bright that the actual emulsion of the film bleeds, bright objects can cause internal reflections inside your camera lens causing lens artifacts from glow to lens flares, the photographer has placed a special "soft filter" over the lens, the photographer has placed vaseline on his lens to give their photo that "soft" look. Simulating this effect in cg is actually pretty straight forward, and helps remove some of that "cg" look from your images.

Figure 1

For example, here's a real photo of my floor lamp (exposure set to 1/125 of a second), notice how the glow from the lamp seemed to creep over the edge of the black bowl.

If I were to make a digital 3d lamp, it wouldn't be enough to just model, texture and place a light source coming from inside the lamp, you'd also want to simulate this glow to make it look more realistic.

Specular Blooms can be added to your cg images in a number of ways. They can be added in 3d through features inside most 3d renderers. They can be added as a post process in compositing packages such as Nuke. In this tutorial, I'll quickly show off a technique for adding them in photoshop.


The underlying process is pretty straight forward, take your image, select the brightest pixels only, blur them, and then add them back to the original image. A more detailed procedure follows...

Take your image, duplicate the image as a layer

Figure 5

Heighten the contrast and reduce the brightness to get just an image that only represents the bright parts of your image (for my test I changed the brightness to -30 and the contrast to 90)

Figure 6

Desaturate the layer (you can experiment with only desaturating it slightly or modifying the hue of this image to achieve colored glows)

Figure 7

Blur it with a "Gaussian Blur" filter (for this test, I used a value of 15)

Figure 8

Note: You can also experiment with the "Lens Blur" filter instead of the "Gaussian Blur", which will give you a more defined defocusing effect similar to a Bokeh effect.

Then set the mode of the layer to linear dodge (this is an additive blend, see my Additive Mode In Photoshop lesson for more discussion on linear dodge and it's relation to an additive mode). Now your have a glow layer sitting on top of your original image. Playing with the layer's Opacity controls how much of the glow is mixed with the original image.

Fifure 9

Here's what the layers look like for this effect...

And here's an experiment with adding color to the blurred layer...

How much to glow (brightness and size) is up to you. For a very soft, angelic feel, a slight glow but a big size may be more appropriate. Or if you're matching a cg element into a real photograph, take a look for visual cues in the photo. How much glow appears in the bright parts of the photo? Then try and match those on your cg object, to help it blend with the photo seamlessly. Good reference material is going to be your bible here, and will help you get the look you want.

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