Chroma Keying – مفتاح الكروما

ما المقصود بمفتاح الكروما ؟!
What is Chroma Keying ?

لابد وأنك شاهدت هذه التقنية تستخدم لوقت
طويل من دون أن تلاحظ ، فأغلب أفلام هوليوود
تستخدم تقنية الكروما ، كيف ؟! هل تعتقد أن
مدرسة هوغوورتس موجودة فعلاً !؟ معظم
الخلفيات كانت مضافة بتقنية الكروما .ولعل أشهر
أستخدام لهذه التقنية هي نشرة الأحوال الجوية .
You’ve probably been watching chroma keyed
video for a long time. Most Hollywood films
have some sort of video compositing. You think
Hogwarts actually exists? I wish! Most of those
backdrops and sets were completely digital.
A common use of green screen is during
weather forecasts.

ما المقصود بمفتاح الكروما ؟!
What is Chroma Keying ?

يقصد بمفتاح الكروما عملية تصوير مشهد على
خلفية ذات لون واحد ثم يتم إزالة هذه الخلفية
بإستخدام برامج الجرافيكس ومعالجة الأفلام ،
واستبدالها بصورة أو مشهد آخر.
Chroma keying is the process of shooting
video on a solid color background and then
digitally removing this background during
the editing/post production process. Then
you can fill in that transparent area with
a different image or video.

إذاً ماسبب شيوع استخدام اللون
الأخضر والأزرق ؟!
Why Are Blue and Green Screens
Common for Compositing?

الأزرق والأخضر في الطرف المقابل للون الأحمر و
البرتقالي المكون الرئيسي لدرجة لون البشرة ، في
الواقع لاتحتوي بشرة الانسان في الحالة الطبيعية
على اللون الأخضر و الأزرق ، بالتالي استخدام اللونين
يضمن عدم تأثر الشخص عند تطبيق تقنية الكروما .
Blue and green are on the opposite end of the
color spectrum from red and orange, the main
colors in skin tones. In fact, skin tones do not
naturally contain any blue or green, so by using
blue or green you’ll have less interference
when keying a person on screen.

The biggest consideration for blue screen vs green screen is the color of foreground costume, objects, etc. It is imperative that the colors in the foreground of your video are not in the same color range as your blue or green screen. If your talent has green in their outfit and they’re against a green screen background, the green parts of the outfit may be removed when you key out the background. If you’re working on a large-scale production against a screen, decide early on the “illegal color range” for any objects in front of the screen!

مالفرق بين اللونين ؟!
What is the difference between
green screen & blue screen?

أن أهم نقطة تحدد متى يتم استخدام اللون الأزرق
ومتى يتم استخدام اللون الأخضر ، هي ألوان
الموضوع أو الأشخاص ولباسهم في المشهد المراد
تصويره .ولكن هناك بعض العوامل الأخرى التي من
شأنها أن تأثر في اختيارك للون مثل :
-الكاميرا المستخدمة . – انعكاس اللون . – إنتشار
استخدام اللون الأزرق . – الإضاءة .
But the choice to use green screen vs blue screen extends beyond just the outfit choice of your talent. Here are a few reasons why you should pick one over the other.

4 Main Differences Between Green Screens and Blue Screens

  • The use of digital cameras. When using a digital camera instead of film, you obtain better results from a green screen shoot because of the cleanness and luminescence of green over blue. Using green instead of blue results in less noise when keying out the footage.
  • Color spill. While the green or blue screens will be removed during editing, some color will “spill” onto the subjects, especially around the edges. This can create a thin line around the person, or make areas like their hairline look odd. Depending on your shoot, color spill can be better or worse depending on the color of your screen. Blue screen tends to have less spill than green, and also happens to be easier to color correct than green. 
  • The prevalence of blue. Subjects and objects are more likely to be blue than green. For example, people are more likely to be wearing blue clothing than green clothing. You get better results when the background color is not heavily present in the subject you are filming (which is why red screens and yellow screens don’t exist).
  • Lighting. Blue screens have a lower luminosity as compared to green screens. This means they are better suited to low-light shoots, but also makes them trickier to light. If you want the blue screen to key properly with the least amount of adjustments necessary you’ll need to set your camera to a higher f-stop than you would were you using a green screen. This also means it is more difficult to shoot a large scene on blue screen as opposed to green.