Lookdeava is an important process to determine the appearance in CG production.In this paper, we will have CGSLAB Naoki Komematsu and Hayashihikar explain the technique of reference shooting, which is important in the looks of live -action synthesis.* This article is a partially re -edited article of CGWORLD282 (February 2022).
記事の目次Texture generation using crossing polarized photography
As an example of applying polarized photography, it is possible to create a high -precision PBR texture including a simple but dark room and light arrays, including a normal map generation in a photometryx tereo.
<5> Precautions for crossing polarized photography
Points for shooting for textures
In the case of texture shooting, the ideal white light source with high coloring of lighting is ideal. Depending on the transmittance of the filter, the polarized filter will drop about 1 EV when installed. In the case of crossing polarization, both the camera and the lighting are attached to both the lighting, so the exposure of about 2 EV is darkened. Since the actual light amount is 1/4, if the same light amount as before installation is required, it is necessary to increase the light source or exposure of the camera to more than 4 times. By making the camera and lighting parallel, the effects of crossing polarization can be maximized. In addition, it is difficult to reflect the shadow by making it parallel, and it is easy to shoot flat. The cross -polarized photography is established by inserting a PL filter between the floodlight and the floodlight part, and shooting in a lighting environment that has polarized light in light. This loses polarization just by sandwiching or reflecting the diffuser after polarizing. Therefore, prepare a dark room covered with a dark curtain as much as possible and shoot in an environment where light is difficult to reflect. If the lighting is too close, the illuminance will come out, so it will be easier to handle as a texture by releasing the lighting to some extent. When taking a reference shooting in the same environment, the camera filter is removed. It is not very suitable for shooting metal reflections, but you can see some unique metals.
Ingenuity when shooting
When shooting a difference at 0 degrees and 90 degrees of crossing polarization, a tripod shooting is required so that the camera does not shift.If you want to shoot with a small shot with less shots, you can automatically rotate the polarized filter of the camera with a motor or the like, or switch two lights with a 90 -degree angle.) You need to.
Light and camera polarized filters can be adjusted by parallel.By adjusting the polarized angle, it is possible to write 360 degrees.
Difference depending on the quality of the filter and the presence or absence of polarization
Difference at 0 degrees (parallel) and 90 degrees (intersection) = Specualal Falence.When a difference is taken from the cross -crossed image, there is a phenomenon in which bluishness appears.This is the difference between filter performance, UV, infrared wavelength, 0 degrees, permeability at 90 degrees, absorption of specific wavelengths, absorption of specific wavelengths, fluorescent and reflective rates.It occurs by etc.In some cases, such as fluorescent colors, the difference in impression color may be reduced by returning this difference to a polarized 90 degree material.For this reason, basically, the result of this bluishness is not a specula color, so it is preferable to use a surcharge as a reference.Depending on the material, it can be used as a roughness map base by turning the images with the saturation into an inverted image.
<6> RAW development of crossing polarization shooting
Free software "DARKTABLE" development
In the case of crossing polarization shooting, we will introduce how to develop and color correction in the free RAW development software DARKTABLE used in the previous HDRI creation article.The best is a development using IDT, but it is not always possible to prepare an IDT with a camera for texture shooting.Since the use as a texture is assumed, it is best to develop a calibration on a color chart excluding the color of lighting.DARKTABLE has a calibration function that matches the color chart, so I would like to use it.The version used is Ver3.6.It is 0.There is no problem with gizmo correction such as mmcolortarget for NUKE.
At the time of crossing polarization, the color chart is taken with the same environment and camera. The camera is taken without a filter, taken by exposure, photographed with crossed polarization 0 degrees, and 90 degrees. Calibration is performed without filter. The calibration settings without filter are applied to the polarized crossing, and only the white balance is reformed at 90 degrees in crossing. This is because the white balance is slightly shifted due to the filter characteristics. This development setting will be applied to 0 degrees and other images with polarized photography. If you use it as an albedo texture after writing, the contrast is stronger due to the characteristics of polarized shooting as it is, so the black point and white point are corrected based on the standard value of the color chart, so that it is more proximal to albedo value. increase. The dark part information may be crushed due to the effects of the camera's dynamic range, iOS sensitivity, noise, etc., and it is recommended to shoot brackets depending on the camera because it also affects color accuracy. In addition, if you take advantage of the occlusion element and shadow such as a fabric, it may be better to correct blackpoints. Since this calibration work depends on the setting of the camera and the lighting environment, if you are not shooting in the same environment, you can get more appropriate results. Keep in mind that it is not always the right result even if the result that is close to albedo value is obtained for simple correction. In particular, depending on the condition of the lighting and the visible angle of the target, the reflection due to brightness and polarized light may not be removed and may not be correct.
Development procedure
Color measurement with NIX
By using Nix Pro Color Sensor, you can measure the unique colors of the object from the diffusion reflection light and get a value close to Albedo.RGB and LINEA (R Lin (R Lin), which are common as a characteristic point.It is also possible to display RGB values not only in RGB but also in AceS / acescg.
<7> Drop the real element into the material
Parameter elements for PBR shader
In order to match the reference groups taken so far and the CG, it is important to drop the real elements into the material. The details of the PHYSICAL-BASED workflow itself will be a huge story, so while referring to the well-organized Substance PBR Guide, we will focus on this look-Dev and explain the points and explanations. To go. The texture of the surface can be seen by the light reflection on the surface of the material. For example, the texture such as paper and pears, the surface is evenly rough, so the highlights look blurred and matte texture. Conversely, the texture such as plastic and glass is not rough and smooth, so the highlights look like a mirror -like texture. In order to reproduce the rough state of this surface, it is inefficient and more likely to be reproduced with a micro -industrial irregularities with a polygon. Efficiency and noise are difficult to come out. And since this rough state reproduces the fact that the surface is randomly uneven, the normal map that is referenced from the texture in the UV space, or the bump that also specifies the uneven level. Similar reproduction is possible with textures such as maps. The display map expresses the unevenness by displacing the vertices. It is possible to express the fine details of the detailed detail level by making a micropoligon by rendering, but the calculation cost will be reduced if only large undulations are targeted.
About base color
It is possible to simply extract a unique color (albedo) of the surface from the image taken using the PL filter.By comparing the color information measured by the NIX introduced earlier, you will be able to obtain a general rule.The image is a comparison with the color measured by NIX.In addition, the color changes slightly depending on the environment such as the base color of the coating, the translucent, and the three -dimensional lighting hit, so it is a good idea to observe it from multiple angles and correct it.
Relationships and reflections between roughness and detail maps
Rafiness, normal / bumps, and displays can define the roughness of the surface, so it is necessary to pay attention to the map creation, so it is better to prepare each based on load and creation costs, as well as the size of the details.prize.The exception of the reflective system is brushed such as fabrics.This is characteristic that the more you go outside, the more you go up and the side, and the straightforward side is black to get the light -skinned scalp color.
Structural color, fluorescent, special material
As an exception, it is necessary to examine and express the color of the material that is not expressed only by the simple color reflection.In the most common case, disk surface such as oil film, DVD, and tambu beetles are polarized into rainbow colors by reflecting specific wavelengths due to fine irregularities on the surface.In addition, special application materials such as special inks and recursion reflective materials will be consulted with a shader artist.
<8> Comparison of reference and rendering results
Easy to adjust the details by holding down the points
The image in the right section is the rendering comparison of the 3D scandata, the texture corrected from the reference image, the HDRI images and the materials of the reference image, and comparing them while paying attention to the contents so far. The setting was about 10 minutes. Actually, we will start with a metal ness map and roughness map. Since bumpers are metal, it is necessary to specify on the metal ness map, but you can check the same color, highlight position and appearance. In this way, if reference shooting and texture shooting are stepped on the fixed procedure, it will be possible to immediately find the matchability in the same lighting, making it easier to understand what adjustments are needed. If the model is adjusted in advance by such a procedure, even in the worst case where HDRI shooting is not possible at the shooting site, it is possible to perform a photorial live -action synthesis by adjusting only writing. Conversely, if you have an appropriate reference image and HDRI, you can also find texture and material color and texture in back calculation. In addition, even if it does not match, it will be possible to limit the cause to only lighting, separated from unique and vast elements such as material parameters.
Material prepared by the procedure so far
Comparison between reference images and rendering results
On the left is a reference image with RAW development using a color profile.Mapping images corrected after polarized shooting as texture to 3D scanned data on the right, writing with HDRI taken with the reference image and white balance, and comparing only the roughness parameters.Adjustable rendering results.In particular, you can see that the highlight position of the wheels match.
The contents of this article are also explained in "CGWORLD FRIDAY" distributed on 1/7 (Fri).If you are interested, please see the following archives.You can see it immediately from the appearance part.
CGWORLD VOL.282 (February 2022 issue)
Special feature: "BANG DREAM!" Anime Series List price: 1,540 yen (tax included) Type: A4 Wide Total Page: 112 Release Date: 2022
詳細・ご購入はこちらからTEXT_CGSLAB (Naoki Komatsu, Hayashihikar) Edit_ Hitomi Komura / Hitomi Komura (CGWORLD), Momoko Yamada / MOMOKO YAMADA