2013年2月13日 星期三

ACES 色彩格式(翻譯文)

ACES - Academy Color Encoding Specification(美國影藝協會色彩編碼標準)

原作者: Nikolai Waldman
原出處:http://colorgrader.net

The ACES system has been created by the Academy of Motion Picture Arts & Science technology committee in order to achieve a more accurate color pipeline, regardless which camera you are using.
ACES 系統是由主掌金像獎的美國影藝協會所主導的一個色彩學規範,目的是達到更精準的色彩工作流程,而將攝影機的影響降低。


The idea is that this pipeline will be manufacturer independent and of course HDR compatible. The ACES color space is based on a theoretically unlimited color space defined in XYZ coordinates. As you can see in the graph below it's larger then DCI.
這個標準最初的理想,排除了製造廠商的影響,同時也支援 HDR 格式(高曝光動態範圍)。ACES 是根據 XYZ 色彩學而來的,理論上是一個不受限制的色彩空間,大家可以用下面 DCI 的示意圖來理解。(DCI 是美國數位電影提倡機構)
It's almost impossible to clip or limit color while grading in ACES.
採用 ACES 色彩空間來調色,不可能受到限制或者是因超標而被切掉。

How does it work?
Each camera is different in the way it captures the image. The idea is to have a common high end target format that each camera maker allows you to render or convert your images into. This allows you to match 2 different cameras types on the same timeline. This way you can copy and paste your grades between them in a simple manner. Once everything is graded in the ACES color space you just convert it to your target, which might be Rec.709.
它的工作原理是什麼?
每一款攝影機都有其詮釋圖像的方式,當初的創意是希望可以將各家攝影機,輸出或者是轉檔成為一種高階的格式。這種格式也可以讓你,針對兩種或者兩種以上的攝影機素材進行校色。校色後標準的影像,才能夠讓調色師使用剪貼、複製的方式來工作,增加了工作的效率。當你在 ACES 色彩格式下工作完畢之後,再輸出成想要放映的規格,例如: Rec.709。

To encode the ACES color space requires advanced math in the IDT stage of the process. The values are traced back to represent what was actually shot. This can be achieved by doing an extensive series of tests of the camera that is used, along with the optical systems. A very accurate picture of the real world light values in the original captured scene can be obtained.
如果想要輸出成 ACES 色彩格式,需要攝影機製造商運算影像的高階數學運算法,這樣才能夠回朔到拍攝當時影像。為了達成這樣的結果,需要一連串針對光學與攝影機的測試,計算出一套非常精準的數據,與攝影機所拍攝出來的影像做比對。

The ACES workflow(ACES 工作流程)



IDT (Input Device Transform)「輸入格式的轉化」(攝影機上的機制)
Sensors should be linear, but many manufacturers add an S-log or Log C curve or some "special" things to the image. The IDT process will restore the real world values.
感光元件上應該呈現的是線性的數值,不過很多製造商將它加上 S-Log 或者是 Log C 甚至於其他特殊的效果來呈現影像。IDT 流程的工作,就是將它還原到原來的數據。

ACES(轉檔成 ACES 色彩格式)
This is the stage where all the grading and compositing is done.
在這個階段完成所有調色與合成的工作。

RRT (Reference Rendering Transform)(相關的影像格式輸出)
In this stage you need to select what the target color space is. Depending on the target color space you will loose information, but this should be nicely handled by the software you are using.
在這個階段,你可以選擇所需要輸出的色彩格式,進行轉換的工作。在運算的過程可能會減損資料量,因此應該要小心的應用軟體來處理這樣的工作。

Implementation(應用)
There are two ways to implement this into your workflow. The first one is to render new files with the ACES color space and the second one is to implement the ACES conversion into the grading software.
有兩種應用的方式:1. 將使用 ACES 製作完畢的檔案,運算成一個獨立的媒體,再套用回原有的工作流程。2. 將 ACES 直接轉換到專業的調色軟體。

If you take a look at REDCINEX-X you can find the ACES color space hidden in the OpenEXR export options. This way you can use any OpenEXR compliant application to work with the ACES color space. The OpenEXR file is no longer a RAW file, so those settings can't be adjusted. On the other hand the ACES color space is so flexible that you won't be limited later in the process unlike if you convert to let's say Rec.709.
如果你看一看 REDCINE-X ,會發現 ACES 色彩格式,被隱藏在 OpneEXR 輸出的選項之下。這樣一來你就可以使用,任何可以接收 ACES 色彩格式的軟體來工作。ACES 格式檔案已經不再是 RAW 檔,因此它不能像 RAW 檔那樣調整。不過,ACES 是如此的具有彈性,它不會像是 Rec.709 一般受到限制。
The second approach is found in DaVinci Resolve 8.1. Here you set your project to work in ACES colorspace and then tell the system what camera the material you work with is shot on. If it's on different cameras you can set this per source-clip. The RAW settings are not used, you can just grade away burned highlights or faulty color temperatures.
在 DaVinci Resolve 8.1 版發現另一個工作訣竅,在這裡你可以將專案設定以 ACES 色彩格式來工作,並且告訴系統影像檔案是由什麼樣的攝影機在什麼條件下所拍攝。如果有多種攝影機所組成的專案,可以針對不同攝影機條件做調校。RAW 檔案的設定無法在這個條件下設定,但是還是可以一樣的調整過高的亮度,以及錯誤的色溫。

I really hope that more manufacturers will implement this as soon as possible since it will make our life so much easier.
由衷的希望會有更多的軟、硬體廠商投入這樣的色彩格式,因為它實在會讓我們的工作輕鬆一點。

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