ADVANCED THEORIES FOR CG LIGHTING

Transcription

1 ADVANCED THEORIES FOR CG LIGHTING 0.1 INTRODUCTION To become skilled at 3D lighting, one must have an understanding of how light works. CG lighting has been established based on rules from cinematography, painting, photography, stage design, and architecture. Until a 3D scene has been lit and rendered, it merely remains nothing more than a bunch of polysurfaces. The qualities of real world lighting, CG lighting types, shadows, lighting techniques, and some basic tips for rendering and layout will be discussed. As a lighting artist, one should be concerned with more than just the illumination of your scene. Scenes should convey an appreciation of the composition and introduce drama.

2 1.1 REAL WORLD LIGHTING A. The Visible Spectrum of Light The visilbe spectrum of light is represented by wavelengths where the shorter waves are near ultraviolet radiation and the larger waves are near infrared radiation. Moving from the shortest to the largest wavelengths, the colors of violet, blue, green, yellow, orange, and red can be seen. All colors within the visible spectrum of light can be created by the additive process and combination of red, green, and blue; also known as, the primary colors of light. That is why all computer graphic programs have RGB inputs to create color and allow ranges between 0 and 255.

3 B. Monitor Calibration One thing to note is that your CG lighting techniques are affected by the calibration of your computer monitor. If your screen is very bright, your scene will most likely be under lit. If your screen is too dim, your scene will most likely be over lit. It is recommended that you calibrate your screen to maximize the full tonal ranges available. In the publishing world, calibrating your screen means to match the color and tones of your printer. In the digital world, calibrating your screen means to adjust the brightness and contrast of your monitor until you see the full range of grays and read the word Calibrate in the image above. Calibrating your screen ensures that you are lighting a scene as effectively as possible. C. Color Balance Professional cinematographers use different films that have been color balanced at a specific color temperature. This dictates which color of light will appear white when filmed and how the other light sources will be tinted. The camera in a 3D software environment does not use film and does not have the color balancing capability that real world film provides. To produce photorealistic renderings, you need to adjust the colors of your computer model lighting to mimic the color balancing effects of film. Determine what type of light each light actually represents and adjust the RGB values appropriately from shades of red, orange, yellow, white, light blue, and dark blue (see the image above). It only takes a small amount of color to begin to tint a surface; therefore, be careful not to make the tinted light too bright. There is no set formula for tinting lights to produce a final look that mimics film. Use your own eye and artistic judgment.

4 D. Behavior of Light Light obeys many rules; however, in CG lighting, only a few rules require understanding: the Inverse Square Law, Law of Reflection, and the Index of Refraction. The Inverse Square Law describes how light fades over a distance. As light travels further from its source, it covers more area; thus, the light loses its intensity. This law is important to understand in 3D programs because this is how real lights behave. In most programs, however, this option is not always turned on by default. As you place lights in your scene, it is recommended that you turn on the decay or attenuation settings. The Law of Reflection states that the angle of reflection equals the angle of incidence, which is measured relative to the surface normal at that point. This is simulated by rendering engines that use raytracing. Raytracing simulates reflections and refractions so that light and color bounces off of objects in an accurate manner (see the reflectance values of real-world materials on the next page). The Index of Refraction (IOR) describes how light bends when it passes through transparent and translucent objects. This behavior determines the level of distortion that you can see on and through that object. The index is never at a value less than 1.0 and can be increased to a value of 2.0 or beyond. As the value increases, the distortion increases (see the refraction values of real-world materials on the next page).

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6 Shadows: play a massive role in describing light adding realism, consistency, relationships, and composition. Shadows are relative areas within a scene where light is absent to a degree. When we place lights, we also must think about the shadows that will be cast. There will be more on this topic later. associated positive and negative effect to take into consideration. Blue light can paint an unhappy or a calm and serene scene; whereas, Color: of light gives you a visual clue to what is happening within red is often used to set a scene. Color of lighting can describe the type of light source, a dangerous tone or a time of day, season, type of weather, and the mood of your passionate scene. Green scene. Color is a sizable consideration in lighting a scene and is associated with peace should be carefully used. and environmental awareness, but can also Yellow to orange light is typical of domestic lighting and blue light refer to greed, envy, or is typical for light coming from the sky. Each color has an nausea. E. Qualities of Light To ensure that your lighting efforts in CG appear convincing, there are several characteristics that make a light source look real and can be described by its intensity, shadow, color, softness, throw, and motivation. Intensity: describes how strong or bright a light source appears. The light with strongest intesity in a scene is known as the dominant or key light and will cast the most noticeable shadows. A light s intensity (its color and multiplier) needs to be adjusted depending on the context of the scene.

7 Softness: of light is widespread in the real world and directly correlates to the sharpness of shadows that are produced. In CG environments and because of default settings, softness does not appear as much as it should; therefore, we tend to see more crisp-edge shadows than you would typically see in real life. Just about every light produces soft-edged shadows. Throw is the manner in which a light s illumination is shaped or patterned by the breaking up of the light through a physical object or by texture map. An example of throw is the light shining through a lampshade, foliage, or even venetian blinds. Motivation: of light is a way to categorize light sources by how they operate within in a scene whether logically or emotionally. Logical lights, also known as Practical Lights, are light sources that represent an actual light source. If you can see a light in the ceiling or see a lamp on an end table, these should be actual logical light sources within the scene. We expect that those components will be a light source. Placing only logical lights within a scene can have uninspiring results; therefore, we need additional lights placed that play an emotional role. These lights are referred to as Pictorial Lights and are placed simply because the effect that is produced is aesthetically pleasing. These lights introduce the drama and the emotional link for your image.

8 1.2 Shadows The Importance of Shadows Shadows are the biggest aesthetic consideration for lighting a scene. Shadows give the human eye a cue to distinguish where a light source is located, what an object is made of, how far away it is, and how it relates spatially to its surroundings. Shadows also add to the 3D nature of a scene. Since our rendering is a 2D image, shadows can be used to show items within a scene that may exist out of frame. Shadows also are a useful tool for establishing the focal point within a scene and for hiding imperfections. By obscuring the areas surrounding an object, you are effectively framing that object as the focal point of the image. The Technical Side of Shadows Shadows are the most computationally intensive part of lighting. Knowing when to turn off shadows from a light source is an important consideration while lighting a scene. Typically, only your main lighting source should cast hard shadows and your intermediate sources should cast softer shadows. Ambient and area lights should have shadows turned off to save on render times. If all light sources have shadow casting turned on, your scene will be visually distracting because the light sources are competing and will also add to your render times. Shadow Saturation If your shadows are too dark, you need to add ambient/fill lights to the scene to lighten them up. Shadows in general are more gray than black; therefore, adding fill lights will make almost black shadows into grayer casts.

9 1.3 Light Types Omni/Point Lights are lights which emit an even distribution of light around a single point. These lights are best used to provide overall fill lighting. Area Lights, such as Rectangular and Linear Lights, provide a more realistic light in that these lights provide the ability to create a light source that has an actual scalable size. The larger the area light or the more area lights that cover an overall area, the softer the shadows will become. The only down side to an area light is that since it more closely represents realistic lighting sources, it can be computationally intensive. Spot Lights cast a focused beam of light confined to a cone that is aimed at a target. This light is the basic building block of lighting a scene because it offers the most controllability. The cone is made up of a hot spot and the falloff ring. The hot spot it the area in which 100% of the light will shine; whereas, the falloff ring it the area in which the light gradually falls off from 100% to 0%. The closer the hot spot to the falloff ring, the sharper the boundary edge of the cone will appear. Adding more distance between the hotspot and the falloff will create a softer edge.

10 Experience Notes: Interior lighting, depending on the scene, requires numerous types of lights set at low multiplier values to simulate logical/actual lighting and additional pictorial lights to add some drama and flare to the shot. If you add just one or two lights to an interior shot, your rendering will appear flat. Real lighting works by giving the scene an even distribution of decaying light over a given area allowing the light to bounce off of materials and surfaces to further light a space. Notice that every interior space you walk into and occupy has a set of lights spread out in a repetitive pattern. Paint colors and materials selected add to the reflective value of space; thus, adding to the amount of light bounced off of a surface. Interior lighting is much about the indirect bounce of light, as well as, the direct light source and target. This image illustrates the final rendered shot of the UNCC Student Union Rotunda. The next image shows the amount of lighting required to create the scene. These images were created within 3D Studio Max; however, the same concepts apply in Rhinoceros using the VRay Plugin (the two images on the right side of the previous slide are the lighting to rendered image comparison within Rhinoceros using the VRay Plugin).

11 Directional/Sun Lights cast parallel light rays in a single specified direction. Directional lights often are used to simulate sunlight. The position of a direction light does not matter; only the rotation angle is crucial. Directional lights are also used for fill lighting also known as ambient light or the general illumination of a scene. Ambient/GI Light is not a light source you place, but is the general/global illumination of a scene created by light reflecting off of other objects. In real life, it is the ambient light that allows us to see into shadowy corners, and in real life the ambient light actually varies based on the objects in which light is reflecting. In computer graphic programs, ambient light cannot be calculated as it occurs in real life; therefore it is generally applied evenly across a scene. The use of ambient light will generally make your scene appear flatter depriving the scene of any depth or contrast. For realistic lighting results, the ambient light tool should be turned off or set to black due to the unrealistic way it applies the illumination across a scene. Adding fill lighting such as low valued point lights or directional lights is the best way to create ambient light.

12 2.1 LIGHTING TECHNIQUES A. Three-Point Lighting The convention of three-point lighting is one that is firmly established in cinematography and is the foundation for CG lighting. Lighting with one source and behind the camera view will always make your rendering look flat. In this system, there are three light types involved and each has a specific function. It should be noted that the threepoint lighting system is not meant to be taken literally. You can have more than three lights in a scene. What is important is that the way in which all the lights are used should fit into the Three-Point Lighting categories: Key, Fill, and Back Lighting.

13 The key/dominant light provides the main illumination in a scene and casts the most obvious shadows. This might be the sun in an exterior scene, the sunlight entering through a window in a daylit indoor scene, or the logical lights in the ceiling of indoor scenes. The Key Light creates the subject s main illumination, and defines the most visible lighting and shadows. It is often set at 15 to 45 degrees to the side (to the left or right and higher) from the camera. Be sure to stop and do text-renders of your key light, which should have a nice balance and contrast between light and dark, and shading that uses all of the grays in between. Your key light rendering should almost look like the final rendering except there are probably pitch black areas or very harsh contrasts visible, which are corrected with the next lighting concepts. The fill light is used to apply indirect lighting to the entire scene and is usually placed on the opposite side of the subject from the key light; thus, it opens up the lighting on the side of the subject in shadow and reduces the shadow intensity. The fill light softens and extends the illumination provided by the key light, and makes more of the subject visible. Typically fill lights consist of point and spot lights and a scene can have several fill lights added to a scene. From the top view, a fill light should come from a generally opposite angle than the key light; however, in elevation, it should be lower than the key light. At most, fill lights can be about half as bright and for more shadowy environments, use only an eighth. Be careful that multiple fills can overlap; therefore, the sum of their values should still fit within the half to eighth range. The backlight/rim light is used to separate the focal piece from the background and to highlight the subject. Essentially, this light creates a bright rim around the object in which you are highlighting. From the top view, add a spot light, and position it behind your subject, opposite from the camera. From the right view, position the Back Light above the subject. Adjust the rim light until it gives you a clear, bright outline that highlights the top or side edge of the subject. Rim lights can be as bright as necessary to achieve the effect. A rim light also usually needs to cast shadows.

14 B. Night Time Moonlight is a neutral gray in color; however, it is best represented from an orangey-beige color to a pale blue color as it approaches its highest point in the sky (Figure 9.09). The best strategy in CG is to make the moonlight the overall color using the orangey-beige to pale blue color and then adding in blue fill lights. Yellow-orange area lights can then be placed to simulate artificial street lights or lighting from behind a window.

15 3.1 TIPS & TRICKS A. Rendering It s never too early to start rendering. As soon as you place your first light, render a quick shot to test its effects on the scene. You lights should be placed relative to the scene and camera view. Obviously, if you have more than one view, you probably will have more than one three-point lighting scheme. Set one light and get it right, then add the others. Start with the key light, then the fill light, and then finally the backlight. With each addition, you will most likely have to tweak the previously placed light to a lower multiplier value. Sometimes it is also necessary to tweak the actual construction of an object to make the lighting have more drama such as chamfering or rounding the edges of an object to reflect/refract light in an interesting way. B. Common Shot Types To ensure that your production sits comfortably within established boundaries of recognizable cinematic conventions, it is useful to know the different shot types. There are five commonly accepted shot types, all used for specific purposes of conveying a scene s intent and have been converted from character shot types to architectural shot types: Wide Shot Often used to show the overall context and set the location, time of day, and season that will run through the other images within the script. Medium Shot Generally is used to show the overall building with the context not so apparent. Medium Close-Up Generally is used to show main features of the building. Close-Up Focuses on one particular area such as the main space of the building or a particular detail. Extreme Close-Up Frames, highlights, and zooms-in on a specific detail from the close-up shot. C. The Rule of Thirds When putting together a shot, the rule of thirds can help ensure a higher level of interest. By thinking of your image divided into three sections vertically and horizontally, you can use these guides to help position your main elements. Positioning objects in the dead center of a frame is unexciting. By placing elements along the imaginary lines of any of the four intersections, you ll immediately have a far more appealing arrangement.