Real-Time Rendering——8.2.3 Color Grading颜色分级

In Section 8.2.2 we mentioned the concept of preferred image reproduction, the idea of producing an image that looks better in some sense than the original scene. Typically this involves creative manipulation of image colors, a process known as color grading.

在第8.2.2节中，我们提到了首选图像再现的概念，即生成在某种意义上比原始场景看起来更好的图像。通常，这涉及到对图像颜色的创造性操作，这一过程称为颜色分级。 

Digital color grading has been used in the movie industry for some time. Early examples include the films O Brother, Where Art Thou? (2000) and Am´elie (2001).Color grading is typically performed by interactively manipulating the colors in an example scene image, until the desired creative “look” is achieved. The same sequence of operations is then re-applied across all the images in a shot or sequence. Color grading spread from movies to games, where it is now widely used [392, 424, 756, 856, 1222].

数字颜色分级已经在电影工业中使用了一段时间。早期的例子包括电影《兄弟，你在哪里？(2000)和Am elie (2001)。颜色分级通常通过交互式地操纵示例场景图像中的颜色来执行，直到获得期望的创造性“外观”。然后，在一个镜头或序列中的所有图像上重复应用相同的操作序列。颜色分级从电影传播到游戏，现在被广泛使用[392，424，756，856，1222]。

Selan [1601] shows how to “bake” arbitrary color transformations from a color grading or image editing application into a three-dimensional color lookup table (LUT).Such tables are applied by using the input R, G, and B values as x-, y-, and zcoordinates for looking up a new color in the table, and thus can be used for any mapping from input to output color, up to the limitation of the LUT’s resolution.Selan’s baking process starts by taking an identity LUT (one that maps every input color to the same color) and “slicing” it to create a two-dimensional image. This sliced LUT image is then loaded into a color grading application, and the operations that define a desired creative look are applied to it. Care is needed to apply only color operations to the LUT, avoiding spatial operations such as blurs. The edited LUT is then saved out, “packed” into a three-dimensional GPU texture, and used in a rendering application to apply the same color transformations on the fly to rendered pixels. Iwanicki [806] presents a clever way to reduce sampling errors when storing a color transform in a LUT, using least-squares minimization.

Selan [1601]展示了如何将颜色分级或图像编辑应用程序中的任意颜色转换“烘焙”到三维颜色查找表(LUT)中。通过使用输入的R、G和B值作为x-、y-和z坐标来应用这样的表，以便在表中查找新的颜色，并且因此可以用于从输入到输出颜色的任何映射，直到LUT分辨率的限制。Selan的烘焙过程始于获取一个身份LUT(将每种输入颜色映射到同一颜色)并将其“切片”以创建一个二维图像。然后，将这个LUT切片图像加载到颜色分级应用程序中，并对其应用定义所需创意外观的操作。需要注意的是，仅对LUT应用颜色操作，避免模糊等空间操作。然后，编辑后的LUT被保存起来，“打包”到三维GPU纹理中，并在渲染应用程序中用于对渲染像素即时应用相同的颜色转换。Iwanicki [806]提出了一种在LUT中存储颜色变换时使用最小平方最小化来减少采样误差的聪明方法。

In a later publication, Selan [1602] distinguishes between two ways to perform color grading. In one approach, color grading is performed on display-referred image data.In the other, the color grading operations are performed on scene-referred data that is previewed through a display transform. Although the display-referred color grading approach is easier to set up, grading scene-referred data can produce higher-fidelity results.

在后来的出版物中，Selan [1602]区分了两种执行颜色分级的方法。在一种方法中，对与显示相关的图像数据进行颜色分级。另一方面，对通过显示变换预览的场景引用数据执行颜色分级操作。虽然以显示为参考的颜色分级方法更容易设置，但对以场景为参考的数据进行分级可以产生更高保真的结果。

When real-time applications first adopted color grading, the display-referred approach was predominant [756, 856]. However, the scene-referred approach has since been gaining traction [198, 497, 672] due to its higher visual quality. See Figure 8.16.Applying color grading to scene-referred data also provides the opportunity to save some computation by baking the tone mapping curve into the grading LUT [672], as done in the game Uncharted 4 [198].

当实时应用程序首次采用颜色分级时，参考显示的方法占主导地位【756,856】。然而，参考场景的方法由于其更高的视觉质量而获得了吸引力【198,497,672】。参见图8.16。将颜色分级应用于场景引用的数据还提供了通过将色调映射曲线烘焙到分级LUT [672]来保存一些计算的机会，就像在游戏Uncharted 4 [198]中所做的那样。 

Figure 8.16. A scene from the game Uncharted 4. The screenshot on top has no color grading.The other two screenshots each have a color grading operation applied. An extreme color grading operation (multiplication by a highly saturated cyan color) was chosen for purposes of illustration. In the bottom left screenshot, the color grading was applied to the display-referred (post-tone-mapping) image, and in the bottom right screenshot, it was applied to the scene-referred (pre-tone-mapping) image. (UNCHARTED 4 A Thief ’s End c /TM 2016 SIE. Created and developed by Naughty Dog LLC.) 

图8.16。游戏《神秘海域4》中的一个场景。上面的截图没有颜色分级。其他两个截图都应用了颜色分级操作。为了说明的目的，选择了极端的颜色分级操作(乘以高度饱和的青色)。在左下方的屏幕截图中，颜色分级被应用于显示参考(色调映射后)图像，而在右下方的屏幕截图中，颜色分级被应用于场景参考(色调映射前)图像。(《神秘海域4:小偷的末日》c /TM 2016 SIE。由顽皮狗有限责任公司创建和开发。)

Before LUT lookup, scene-referred data must be remapped to the range[0, 1] [1601]. In the Frostbite engine [497] the perceptual quantizer OETF is used for this purpose, though simpler curves could be used. Duiker [392] uses a log curve,and Hable [635] recommends using a square root operator applied once or twice.Hable [635] presents a good overview of common color grading operations and implementation considerations.

在LUT查找之前，场景引用数据必须重新映射到范围[0，1] [1601]。在frosty engine[497]中，感知量化器OETF用于此目的，尽管可以使用更简单的曲线。Duiker [392]使用对数曲线，Hable [635]建议使用一次或两次平方根算子。Hable [635]很好地概述了常见的颜色分级操作和实施注意事项。

Further Reading and Resources

进一步阅读和资源

For colorimetry and color science, the “bible” is Color Science by Wyszecki and Stiles [1934]. Other good colorimetry references include Measuring Colour by Hunt [789] and Color Appearance Models by Fairchild [456].

对于色度学和颜色科学，“圣经”是Wyszecki和Stiles [1934]的颜色科学。其他良好的色度学参考包括Hunt [789]的测量颜色和Fairchild [456]的颜色外观模型。

Selan’s white paper [1602] gives a good overview of image reproduction and the “scene to screen” problem. Readers who want to learn still more about this topic will find The Reproduction of Colour by Hunt [788] and Digital Color Management by Giorgianni and Madden [537] to be excellent references. The three books in the Ansel Adams Photography Series [9, 10, 11], especially The Negative, provide an understanding of how the art and science of film photography has influenced the theory and practice of image reproduction to this day. Finally, the book Color Imaging: Fundamentals and Applications by Reinhard and others [1480] gives a thorough overview of the whole area of study.

Selan的白皮书[1602]很好地概述了图像复制和“场景到屏幕”的问题。想要进一步了解这一主题的读者会发现Hunt的《色彩再现》[788]和Giorgianni和Madden的《数字色彩管理》[537]是很好的参考资料。安塞尔·亚当斯摄影系列的三本书[9，10，11]，尤其是底片，提供了对电影摄影的艺术和科学如何影响图像复制的理论和实践到今天的理解。最后，赖因哈德和其他人写的《彩色成像:基础和应用》[1480]一书对整个研究领域进行了全面的概述。