16-10-17【摄影机】ALEXA SXT/SXR 常见问题解答（第二期）
ALEXA SXT/SXR 常见问题解答（第二期）
本期将为您提供以下有关ALEXA SXT/SXR 常见问题的解答：
影像传感器模式、录制格式、发行格式
1. 能解释一下影像传感器模式、录制格式和发行格式有什么区别吗？
2. ALEXA SXT/SXR摄影机支持哪些影像传感器模式？
3. 怎么变成6:5了？变形宽银幕不是应该用4:3吗？
4. 我怎么知道哪些镜头支持片门全开影像传感器模式？
5. ALEXA SXT/SXR摄影机支持哪些录制格式？
6. ProRes 3.2K对我有什么用？
ProRes 4K
1. 影像传感器只有3.4K成像单元，是怎么在机内录制出ProRes 4K画面的？
2. 以前说只有下采样对画质无影响，为什么现在上采样也可以用了？
3. ProRes 4K UHD和ProRes 4K Cine有什么区别？
4. 为什么ProRes 4K Cine录制格式比4K DCI发行格式的扫描线更多？
ALEXA影像传感器的现在和未来
1. 在“摩尔定律”之下，ALEXA影像传感器是不是已经过时了？
2. 你们有在开发新型影像传感器吗？
影像传感器模式、录制格式、发行格式
1. 能解释一下影像传感器模式、录制格式和发行格式有什么区别吗？
影像传感器模式定义了宽高比，它决定影像传感器上那一部分区域的图像会被记录下来。录制格式决定机内文件都记录了哪些数据。所以尽管影像传感器模式能够决定使用影像传感器上多少数量的像素，这些像素仍然能够通过直接记录、上采样或下采样的方式来创建其它录制格式。发行格式代表最终的产品将以什么形式展现给消费者。在输出最终的发行格式之前，通常在后期处理时就要对录制格式先进行上采样或下采样、裁切、重构图或旋转处理。
举例来说：选择ALEXA SXT的16:9影像传感器模式允许之后进一步选用多种录制格式。4K UHD录制格式意味着读取影像传感器上16:9成像区域的数据后，先在机内上采样为4K UHD画面，然后才录制下来。在这个例子中，录制格式是4K UHD，发行格式同样也是4K UHD。
2. ALEXA SXT/SXR摄影机支持哪些影像传感器模式？
ALEXA SXT/SXR摄影机支持4种影像传感器模式：16:9、6:5、4:3和片门全开。以下是4种影像传感器模式的简要示意图以及各自最常用到的录制格式。
16:9影像传感器模式常用于以HD或4K UHD格式输出的电视节目。16:9 HD录制格式使用影像传感器上2.8K的成像区域，16:9 4K UHD录制格式使用影像传感器上3.2K的成像区域。从16:9 2.8K影像传感器模式中还可提取出16:9 2K Cine录制格式。
6:5影像传感器模式（包括之前的4:3裁切模式）是专为变形镜头准备的。因为变形镜头压缩2.39:1画面采用的压缩系数是2，所以影像传感器上成像区域的宽高比是1.195:1，也就是6:5。
4:3影像传感器模式比6:5模式略宽，高度相同。宽高比为1.85:1（虚线所示）的电影如果视觉特效吃重，都喜欢用4:3拍摄，这样视觉特效团队就可以充分利用1.85:1画面之外的更多靶点做镜头跟踪，或是上下调整构图。
片门全开模式能够利用ALEXA摄影机影像传感器的全部成像区域。2.39:1或1.85:1的电影越来越喜欢用这种模式，宽高比画面之外的额外区域有利于做镜头跟踪或是上下调整构图。上采样到4K Cine录制格式时，这些额外的像素可确保获得最佳的采样效果。
3. 怎么变成6:5了？变形宽银幕不是应该用4:3吗？
4:3是胶片时代的产物，现在使用它最多的是视觉特效大片。变形镜头用不到4:3那么宽的画面，因为变形镜头会将2.39:1画面的宽度压缩一半，最终只用到影像传感器上1.195:1的成像区域，约等于1.2:1也就是6:5。在4:3模式下用变形镜头拍摄，后期制作时必须多做一个步骤，先裁掉画面左右的部分像素，用6:5模式就避免了这个问题。
我们在ALEXA XT SUP 11中就加入了这种模式，不过当时取名叫“4:3裁切模式”，其实与ALEXA SXT/SXR 6:5影像传感器模式是一回事。
4. 我怎么知道哪些镜头支持片门全开影像传感器模式？
我们在ALEXA官方网页的TOOLS区域为你准备了一个新的在线工具：ARRI Lens Illumination Guide（ARRI镜头照度指南）。
每一支镜头中央区域的成像都比四角的更亮：这被定义为镜头的照度。要搞清楚某支镜头在片门全开模式下的表现如何，首先选好影像传感器模式和画框线，然后选择镜头型号、焦段、光圈和对焦设定，这些参数都会影响镜头照度。最终的画面会显示出从中心到四角的照度衰减趋势，以及会不会出现暗角。我们已经为这个数据库录入了绝大部分ARRI镜头的实测数据，但我们的人仍在继续拍摄测试板，所以还会陆续补充新的测试结果。查询的结果不止能在电脑屏幕上观看，也可以存储为JPEG图片。
各支镜头的测试结果有高低差异，这很正常。其中一支镜头的成绩相当好（虽然还没来得及录入数据库），那就是我们的Ultra Wide Zoom UWZ 9.5 - 18超广角变焦镜头。大多数焦段小于18 mm的定焦镜头都无法驾驭片门全开模式，但这支UWZ的像圈设计得很大，真正解决了在片门全开模式下如何拍摄广角画面的难题。
5. ALEXA SXT/SXR摄影机支持哪些录制格式？
ALEXA SXT/SXR摄影机的ARRIRAW录制性能就和XT摄影机一样：16:9 ARRIRAW最高120 fps；6:5 ARRIRAW最高90 fps；4:3 ARRIRAW最高96 fps；片门全开ARRIRAW最高75 fps。
ALEXA SXT/SXR摄影机可录制HD、2K Cine、3.2K、4K UHD或4K Cine分辨率的ProRes，而且由低到高支持从ProRes 422到ProRes 4444 XQ的所有编码。
另外，我们正在研究很多改进以及新的录制格式，所有的测试结果达到我们的要求之后就会发布。
6. ProRes 3.2K对我有什么用？
ProRes 3.2K是从ALEXA XT SUP 11起就出现的录制格式。我们看到一些制作项目用ProRes 3.2K拍摄，然后在后期制作时上采样到4K UHD。我们也发现一些项目先用ProRes 3.2K拍摄，输出时却选择HD或2K，因为3.2K给了他们缩放、重构图、旋转或做镜头稳定的额外图像空间。事实上，在我们的在线工具ARRI Frameline Composer（ARRI画框线生成器）中就专门为这种应用方式提供了一个预设，名叫“1.78:1, 2.8K inside 3.2K”（3.2K中的1.78:1/2.8K）。你可以在ALEXA官方网页的TOOLS区域中找到ARRI Frameline Composer，用来做你自己的画框线非常好用。
ProRes 4K
1. 影像传感器只有3.4K成像单元，是怎么在机内录制出ProRes 4K画面的？
ALEXA SXT/SXR摄影机利用ALEXA 65电子系统富余的处理能力在机内执行了轻微的上采样操作，支持采样为ProRes 4K UHD或ProRes 4K Cine。像素数量只是众多影响4K画质的参数的其中一项，ALEXA的最佳全局画质搭配全新的图像处理链，再加上1.2x的上采样系数相对较小，并未动摇电影人最为看重的ALEXA画质基础。
2. 以前说只有下采样对画质无影响，为什么现在上采样也可以用了？
如果要简短一点回答就是：究竟哪些因素对于呈现好的画面最重要，我们的理解已经发生了改变。技术在进步，ALEXA的全局画质又足够好，所以上采样的效果非常理想。
从理论上说，如果你看重黑白测试标板的分辨率高低，你就应该下采样，也就是说影像传感器的像素要大于你的录制格式。然而，图像不仅仅关乎分辨率，请允许我在这里引用导演兼摄影师罗德尼•卡特（Rodney Charter）的一句话：电影制作不是一门科学。在某些人眼中，似乎摄影师的工作就是没日没夜地拍摄黑白测试标板，分辨率成了决定画质最重要的量化参数。但事实不是这样的，其中有太多关联因素，如果只从一个角度（例如分辨率）来判断、做决定实在太疯狂了。有很多参数，比如动态范围、噪点、肤色还原以及去拜尔质量都要比分辨率重要得多。
随着技术的进步，去拜尔算法的提升也特别特别大，电子系统性能越来越强，我们对机内图像处理和上采样算法也有了大量新认知。我们有一整个部门专门埋头钻研色彩科学，探索一切可能的新技术，这种专注力非常非常关键，他们开发出我们现在使用的机内上采样算法质量太好了。
所以我们愿意在某些地方运用这种轻微的上采样处理，从影像传感器上的3.2K采样成素材文件里的3.8K（= 4K UHD）。在ARRI内部，其实有许多成员曾经反对这种做法，但是我们知道光有理论还不够，只有通过测试才能知道真相，而最终的测试结果非常棒。事实上，我们将自己的上采样图像和其它品牌摄影机的“原生”4K画面放在一起比较，谦虚点说吧，我们的图像品质和对方的一样好。测试结果让ARRI公司所有人都心悦诚服，所以现在大家能在ALEXA上用到这种柔和的1.2x上采样处理，获得这么出色的画面。现在我们对需要录制4K的用户终于有了个交待，那就是4K UHD或4K Cine。
3. ProRes 4K UHD和ProRes 4K Cine有什么区别？
ProRes 4K UHD录制格式（3840 x 2160, 16:9）是为4K UHD TV发行格式（3840 x 2160, 16:9）所准备的。ProRes 4K UHD的画面来自于16:9影像传感器模式的3200 x 1800成像区域，这些数值与AMIRA和ALEXA Mini的完全一致。
ProRes 4K Cine录制格式（4096 x 2636, 1.55:1）是为4K DCI影院发行格式封包（4096 x 2160, 1.9:1）所设计的。ProRes 4K Cine的画面来自于片门全开影像传感器模式的3414 x 2198成像区域，是ARRI摄影机产品线中ALEXA SXT/SXR摄影机独享的录制格式。
以上两种格式都应用了同一套高品质的1.2x上采样算法。
4. 为什么ProRes 4K Cine录制格式比4K DCI发行格式的扫描线更多？
4K Cine录制格式的分辨率是4096 x 2636，比4K DCI发行格式封包（4096 x 2160）要多出477线，这些在图像上下边缘留出的额外区域是为上/下平移重新构图或者制作视觉特效时放置镜头跟踪靶点所准备的。录制格式故意保留比发行格式需求更多的内容，从而为后期制作提供多一点自由度，这就是一个明显的例子。
ALEXA影像传感器的现在和未来
1. 在“摩尔定律”之下，ALEXA影像传感器是不是已经过时了？
首先，我们认为“摩尔定律”并不适用于影像传感器技术。
其次，ALEV 3型影像传感器的成像单元设计独一无二。这套设计为ALEXA带来宽广的动态范围和高光处理性能，它的灵敏度、低噪点水平和自然美丽的肤色还原都是顶级的，它的图像处理链经过长时间考验并且还在不断进步。到目前为止，我们还没有发现任何一款有能力挑战它的机型，因此我们认为现在这套技术方案非同一般。
2. 你们有在开发新型影像传感器吗？
当然了，毕竟我们是一家摄影机制造商。然而，只有像ALEXA一样能贡献更好的全局画质时，我们才会在新产品中使用它，要知道ALEXA的画质难逢敌手，即便是我们自己也很难超越。我们新推出的ALEXA机型ALEXA SXT是一款非常出色的设备，专为现在的专业应用量身配备了许多新特性。
有关ALEXA SXT的更多信息，您可点击查看以下链接：
ALEXA SXT/SXR Frequently Asked Questions (Part II)
This part of ALEXA SXT/SXR frequently asked questions will provide you the solutions for the following categories:
Sensor Modes, Recording Formats, Distribution Formats
1. Can you explain the differences between sensor modes, recording formats and distribution formats?
2. What sensor modes are supported by ALEXA SXT/SXR cameras?
3. 6:5? What the hell? I thought you needed 4:3 for anamorphic?
4. How can I know which lenses will work in Open Gate sensor mode?
5. What recording formats are supported by ALEXA SXT/SXR cameras?
6. Why would I want to shoot ProRes 3.2K?
ProRes 4K
1. How can you record ProRes 4K in-camera when the sensor has 3.4K photosites?
2. I thought you had to over-sample to get a good image. Why is up-sampling now OK?
3. What is the difference between ProRes 4K UHD and ProRes 4K Cine?
4. Why does the ProRes 4K Cine recording format have more lines than the 4K DCI distribution format?
Current and Future ALEXA Sensors
1. Isn’t the ALEXA sensor getting past its prime in terms of Moore's Law?
2. Are you working on a new sensor?
Sensor Modes, Recording Formats, Distribution Formats
1. Can you explain the differences between sensor modes, recording formats and distribution formats?
A sensor mode defines an aspect ratio, which defines the area on the sensor that is being captured. A recording format define what is being recorded in-camera in the file. So while a sensor mode defines a certain number of photosites on the sensor, those may be recorded straight or up or down-sampled to create the recording format. A distribution format defines how the final product is delivered to the consumer. The contents in the recording format is often up or down-sampled, cropped, repositioned or rotated in post before it becomes the final distribution format.
An example: Choosing a 16:9 sensor mode on the ALEXA SXT allows the further choice of various recording formats. Choosing the 4K UHD recording format means that a 16:9 area from the sensor is read out, up-sampled in camera to a 4K UHD image and then recorded. In this case the recording format of 4K UHD is the same as the distribution format of 4K UHD.
2. What sensor modes are supported by ALEXA SXT/SXR cameras?
ALEXA SXT/SXR camera support four sensor modes, 16:9, 6:5, 4:3 and Open Gate. Below is a basic guide to those sensor modes and the most often used recording formats.
The 16:9 sensor mode is most often used for TV shows that need an HD or a 4K UHD deliverable. For the 16:9 HD recording format, 2.8K from the sensor are used. For the 16:9 4K UHD recording format, 3.2K from the sensor are used. The 16:9 2.8K sensor mode can also be used to derive a 16:9 2K Cine recording format.
The 6:5 sensor mode (formerly 4:3 Cropped also) is used exclusively for any shooting with anamorphic lenses. Since anamorphic lenses squeeze the 2.39:1 image by a factor of 2, the resulting area used on the sensor has a 1.195:1 aspect ratio, which is 6:5.
The 4:3 sensor mode is as tall but slight wider than 6:5. Shooting 4:3 has become popular for VFX heavy feature films in the 1.85:1 aspect ratio (dotted line). The area above and below the 1.85:1 image is used for placing VFX markers and/or for up/down repositioning.
Open Gate uses the full sensor area of the ALEXA camera. This format has become popular for those shooting feature films in 2.39:1 or 1.85:1. The extra area above and below the image is used for placing VFX markers or for up/down repositioning. The extra resolution provides the best result when up-sampling to a 4K Cine recording format.
3. 6:5? What the hell? I thought you needed 4:3 for anamorphic?
The 4:3 aspect ratio was originally left over from the film days, and now has been put to good use by VFX-heavy feature films. Anamorphic lenses don't actually need the full width of the 4:3 image; they squeeze the 2.39:1 image by a factor of 2, and the resulting area used on the sensor has a 1.195:1 aspect ratio, which is roughly 1.2:1, which is 6:5. When shooting with anamorphic lenses in 4:3 sensor mode, you always have to crop the extra area to the left and right of the image in post, an extra processing step that is avoided by shooting in 6:5.
In the ALEXA XT cameras with SUP 11 we have introduced this mode, however, there it was still called '4:3 Cropped', but that is the same thing as the ALEXA SXT/SXR 6:5 sensor mode.
4. How can I know which lenses will work in Open Gate sensor mode?
We have a new online tool in the TOOLS section of the ALEXA web pages, the ARRI Lens Illumination Guide.
Every lens is brighter in the center than in the corners: that is called the lens illumination. To get a feeling for how lenses perform in Open Gate, you first choose a sensor mode and what framelines you want to check out. Then you choose a lens model, focal length, iris and focus setting, which are all parameters that affect lens illumination. The resulting image will show how the illumination falls off towards the corners and if you get vignetting. We have most ARRI lenses in there already, but the guys are still shooting, so we will add more lenses as time goes by. You can see the result right there on the screen or save the images as a JPEG file.
Some lenses do better and others worse. One of the lenses that does really well (even though it is not in yet) is our Ultra Wide Zoom UWZ 9.5 - 18; it was designed for a large image circle. Most primes under 18 mm have an issue shooting Open Gate, so the UWZ really solves the question of how to shoot wide angle with Open Gate.
5. What recording formats are supported by ALEXA SXT/SXR cameras?
ALEXA SXT/SXR cameras can record ARRIRAW just like the XT cameras could: 16:9 ARRIRAW up to 120 fps, 6:5 ARRIRAW up to 90 fps, 4:3 ARRIRAW up to 96 fps and Open Gate ARRIRAW up to 75 fps.
ALEXA SXT/SXR cameras can record ProRes in HD, 2K Cine, 3.2K, 4K UHD or 4K Cine. And they can do that with all high-end ProRes flavors from ProRes 422 to ProRes 4444 XQ.
In addition, we are working on a number of improvements and new recording formats, which we will announce once all tests have been concluded to our satisfaction.
6. Why would I want to shoot ProRes 3.2K?
ProRes 3.2K is a recording format we already have in ALEXA XT cameras with SUP 11. We are seeing some productions that shoot ProRes 3.2K and do a post up-sample to 4K UHD. But we are also seeing some productions shooting ProRes 3.2K when they want an HD or 2K deliverable. The 3.2K gives them extra space around their image for resizing, reframing, rotating or stabilizing. In fact, in our online ARRI Frameline Composer we now have a preset for exactly that purpose, called '1.78:1, 2.8K inside 3.2K'. You can find the Frameline Composer in the TOOLS section of the ALEXA web pages, it is a great tool for making your own framelines.
ProRes 4K
1. How can you record ProRes 4K in-camera when the sensor has 3.4K photosites?
ALEXA SXT/SXR cameras use the extra horse power of the ALEXA 65 electronics to perform a mild up-sample in-camera, either to ProRes 4K UHD or to ProRes 4K Cine. The number of photosites are only one of many parameters that form a quality 4k image. The combination of ALEXA's best image overall image quality with the new processing chain and the relatively small up-sample factor of 1.2x preserves the fundamental qualities of the ALEXA picture that filmmakers so love.
2. I thought you had to over-sample to get a good image. Why is up-sampling now OK?
The short answer is that our understanding of what is important to create a good looking image has changed, technology has advanced and the ALEXA overall image quality is so good, that it up-samples beautifully.
Here is the long answer:
Scientifically, if you want the best resolution of a black and white test chart, you should over-sample, i.e. have more photosites on the sensor than in your recording format. However, an image is not just resolution and, if I may quote Rodney Charters here, movie making is not a science project. Some people talk as if all a cinematographer does all day is shoot black and white charts, and resolution is the most important image quality parameter. But that is not the case, and in fact there is so much more to it that it is crazy to look at just one aspect of the image, i.e. resolution, and make all decisions based on that. Parameters like dynamic range, noise, skin tone reproduction and the quality of the debayer are actually much more important than just resolution.
And the technology has changed. Debayering has gotten much, much better, the electronics are much more powerful and we have learned a lot about in-camera image processing and up-sample algorithms. We have a whole color science department that continually pushes the envelope of what is possible, and that is very, very important. They came up with our in-camera up-sample algorithm which is just fantastic.
So at some point we entertained the idea to do this mild up-sample from 3.2K photosites on the sensor to 3.8K (= 4K UHD) pixels in the file. And there were many inside ARRI who were opposed to that. But we have learned that the theory will only get you so far and at some point the only way to really know is to shoot a test. And the results looked great. In fact, we compared it to some 'native' 4K images from other cameras and our up-sampled images looked as good, if not better. That convinced everybody within ARRI that it's possible to do a mild up-sample of 1.2x and still get the beautiful images everyone is used to from ALEXA. Now we have an answer for those who need to record a 4K image, be that in 4K UHD or 4K Cine.
3. What is the difference between ProRes 4K UHD and ProRes 4K Cine?
The ProRes 4K UHD recording format (3840 x 2160, 16:9) is designed for a 4K UHD TV distribution format (3840 x 2160, 16:9). ProRes 4K UHD is derived from 3200 x 1800 photosites on the sensor in 16:9 sensor mode. These are the same values as used in AMIRA and ALEXA Mini.
The ProRes 4K Cine recording format (4096 x 2636, 1.55:1) is designed for the 4K DCI cinema distribution format container (4096 x 2160, 1.9:1). ProRes 4K Cine is derived from 3414 x 2198 photosites on the sensor in Open Gate sensor mode. ProRes 4K Cine is unique to the ALEXA SXT/SXR cameras in ARRI's line of cameras.
Both formats use the same high quality 1.2x up-sample filter.
4. Why does the ProRes 4K Cine recording format have more lines than the 4K DCI distribution format?
The 4K Cine recording format contains 4096 x 2636 pixels, which is 477 lines more than the 4K DCI distribution container (4096 x 2160). This extra area above and below the image is designed for up/down image repositioning or placing of VFX markers. This is an example where the recording format purposefully contains more than is needed in the distribution format to provide for some added flexibility in post.
Current and Future ALEXA Sensors
1. Isn’t the ALEXA sensor getting past its prime in terms of Moore's Law?
First, we don't think that Moore's Law is applicable to sensor design.
Second, the photosite design of the ALEV 3 sensor is unique. That design provides the great dynamic range and the highlight handling that the ALEXA has, it provides the sensitivity, the low-noise floor and the beautiful skin tones. This sensor design combined with the ever improving processing chain has stood the test of time. So far we have not seen any camera that comes close, so we think we have a really unique piece of technology here.
2. Are you working on a new sensor?
Of course we are working on a new sensor, we are a camera manufacturer. However, we're only going to release it in a product if we can achieve better overall image quality as ALEXA. And the image quality of the ALEXA is really hard to beat, even for us. The ALEXA SXT is a fantastic piece of gear with many new options tailored for today's professional productions.
For more information of ALEXA SXT, please click the following link：
ALEXA SXT/SXR Frequently Asked Questions (Part II)
ALEXA SXT Workflow: Codex Software Update