Light Diffusion - Guillermo Algora - Visual Effects Compositor

Guillermo Algora
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Glow / Bloom:

Sometimes, both terms are used interchangeably depending on context or technique. However, in terms of Compositing and VFX, I would like to make a clear distinction between them.


At this point I would like to make a distinction with Bloom. At least from where I see it, I will refer to glow as the natural phenomenon that occurs in the real world. Glow is related to the light emanating from a light source (physically) in the scene, whereas Bloom refers to a more specific effect where bright areas in an image bleed out, creating a hazy overflowing light. It can be caused by limitations of lenses or sensors capturing very bright light.

Therefore, we can describe glow as a soft, diffused illumination around bright areas or light sources in an image, often presenting variations in color, intensity and falloff patterns and characterized by their smooth transition from light to dark. The correct handling of this effect can help to enhance the sense of depth and dimensionality in the image.

On the other hand, bloom refers to a more pronounced and exaggerated spread of light around bright areas, often more localized; whereas glow refers to a general property that has its origin in the physical scene itself.

Bloom, from this perspective, refers to the combined effect of bright light dispersion on the boundaries of contrasting image areas, which originates in the optical system.

The effect appears when there is a lot of light. More light in bright areas of the scene makes the effect more visible - especially in cases of slightly or even heavily overexposed images - and it is best observed along the borders of contrasting objects as the light penetrates the neighbouring dark areas, against which it becomes clearly visible.

Although often present in every image, it is particularly prominent when shooting with wide apertures or when capturing scenes with strong back-lighting. It is also more exaggerated in vintage lenses and certain special optical filters.

Furthermore, bloom on digital sensors is not the same as on film. On the sensor we deal with a simple optical effect, whereas on film, with a more complex effect that derives from the optics but then possibly distorted and amplified in the multiple layers of the photographic emulsion.

A glint is a quick flash of highlights appearing on reflective surfaces (metal, water, etc.).
It is intrinsically related to both the reflectivity of the surface and the angle of incidence, as it only happens only when looking at the surface within the right angle.

Glint is an effect that is alive - i.e. not permanent - any little alteration in the angle should produce a modification or disappearance of the effect. However, a realistic glint "fades", albeit rather quickly, but not just appear - disappear.

Halation refers to a phenomenon observed in celluloid film, never in digital cinema, where intense light scatters within the film, leading to a secondary exposure. Color film usually has three layers of differently color sensitive emulsion, followed by an anti-halation backing. The red layer being the closest to it - followed by the green layer- absorbs any residues of reflected light that bounces back from the back of the glass and that not have been successfully absorbed by the anti-halation backing. Therefore, this effect predominantly impacts the red layer of the film, with some influence on the green layer, and results in a characteristic reddish-orange glow around the light source.
Diffraction and Halos:

As we will see later on, diffraction also has its effects on image sharpness, but this time we will focus on its effect to the perception of light.
Diffraction is the interference or slight bending of waves (in this case, light) around the corners of an obstacle or through an opening (as the lens aperture). The smaller the opening, the larger the diffraction. Due to this, diffraction is more prominent on small apertures (high f-stop) and at low focal lengths, often diminishing as the diameter of the opening increases, either by a lower f-stop or a higher focal length.

Focal length is related to the diameter of the opening, since the formula is: f-stop = the focal length / diameter aperture. In other words,
if you keep the f-stop the same, say at f/22, and you change the focal length, from say 100mm to 50mm, the diameter of the aperture drops in half. Hence, lower focal lengths (at the same f-stop) create smaller openings, so you should see more diffraction at shorter lengths.
Diffraction spikes:

This effect occurs when light passes into your camera through a small opening (a small aperture at a low focal length). It bends around the edges of the blades and creates the "star" look. The number of rays from each starburst is related to the number of aperture blades in your lens. The more blades your lens has, the more rays of "starburst" are possible. However, keep in mind the following phenomenon: the number of starbursts is double the number of aperture blades if there are an odd number of blades and equal to the number of blades if the lens contains an even number of blades. This is due to: a straight edge causes diffraction in the direction perpendicular to that edge. When two straight edges are parallel to each other their diffraction patterns will overlap (point along the same line) - making it look like there is just one, but possible slightly brighter.

Diffraction can also be involved in the perception of halos. As the light diffracts through the small opening, the wavefront gets reshaped. These diffracted waves then travel outwards. When they encounter each other, they can interact in a phenomenon called interference. The intricate dance of constructive and destructive interference creates alternating bright and dark regions around the light source, giving shape to the halos.

At the same time, the relative size of the wavelength and the diffracting object matters. This dependence on wavelength is why we see color in diffraction halos. Different colors of light have different wavelengths, and therefore diffract at slightly different angles. This creates a separation of colors within the halo, with shorter wavelengths (like blue) appearing closer to the center and longer wavelengths (like red) appearing further out.

Atmospheric elements such as water droplets or ice crystals can produce halos due to diffraction, refraction and reflection. Therefore, it is good to keep in mind that if our compositions is set within a dense atmospheric setting, we might take into consideration the proliferation of more halos, as well as more freedom to increase the amount of them creatively if desired.
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