為何 HDR 影像看起來如此鮮明？(Why HDR image looks so vivid?)
文/ Jacky Liu
2016/07/31
HDR 影像 High-dynamic-range imaging (HDRI) 較 LDR 影像 Low-dynamic-range imaging (LDRI) 有更高的動態範圍(Dynamic range)，也就是在單一 Channel 中更精細的記錄影像內容。 Comparing with LDR image (Low-dynamic-range), HDR image (High-dynamic-range) has larger dynamic range, which means that HDR image can record image more precisely. 乍看之下「 HDR 影像格式比起 LDR 有更精細記錄影像的能力」似乎就是其影像更加鮮明的原因，不過這樣的想法存在著問題。
At first glance, “HDR image format has more precise recording ability than LDR image” seems to be the reason why HDR image looks so vivid. However, it contains logically defect.
查看 Wikipedia HDR 頁面後，會發現有更精細的記緣能力只說了一半的故事，另一半同等重要的是顯示，由於傳統顯示器無法完全顯示HDR影像的高對比度，HDR影像必需經過 Tone mapping ，把高動態範圍映射到較窄的動態範圍上。不過在同樣的傳統顯示器上，為何 HDR 影像經過 Tone mapping ，看起來還是比 LDR 影像來的鮮明？
After look up Wikipedia HDR pages, you will find that “more precise recording ability” is only half the story. Another equivalently important part is image display. Because traditional monitor cannot fully display high contrast of HDR image, HDR image has to be apply tone mapping before display. (Mapping higher dynamic range to lower dynamic range.) However, why did HDR image after tone mapping is still more vivid than LDR image in the same traditional monitor?
上述的疑問推翻了 HDR 影像只因為有更廣的記錄能力，而看起來較鮮明的說法(因為在顯示時，HDR、LDR影像都在同樣的窄對比值域上)。
The question above disprove that HDR image looks more vivid just because it has wider recording ability. (Because HDR and LDR image are on the same display dynamic range on monitor.)
其實 HDR 影像鮮明的秘密就藏在拍攝時！
Actually, the secret of vividly HDR image could be found if we observe what camera did when it take a shoot!
一般的攝影設備，如果想呈現影像暗部的細節，就必需增加曝光，但是反之，為了呈現暗部而增加曝光卻會犧牲亮部細節。會有這樣的兩難是源於攝影設備有其拍攝的動態範圍，例如 High-end DSLR camera (contrast 28500:1) 還是無法完全捕捉到和人眼動態時的動態範圍一樣廣(1000000:1)。
If we want to reveal detail in the darkest shadow, we have to adjust the exposure value(EV) higher when we are using a commercial camera. But in the meanwhile, the detail of brightest part will despair. These dilemma is causing by the dynamic range of a camera. For example, even a high-end DSLR camera (contrast 28500:1) cannot capture image with the same dynamic range of human eyes in dynamic (1000000:1).
為了重現如人眼般的動態範圍，可以透過結合多張 LDR 影像來合成一張 HDR 影像，也就是說因為在「記錄」影像時，不用特別的犧牲亮部或暗部，HDR 影像在拍攝時記錄了完整的動態範圍，而這就是 HDR 影像看起來如此鮮明的關鍵！
To reproduce eyes-like dynamic range, we could combine several LDR image to generate one HDR image. For this, we do not need to scarify detail of bright or dark part of image. HDR image record complete dynamic range when we take a shoot. This is why HDR image looks so vivid!
參考資料：
本著作係採用創用 CC 姓名標示-非商業性-相同方式分享 4.0 國際 授權條款授權.
2016/07/31
HDR 影像 High-dynamic-range imaging (HDRI) 較 LDR 影像 Low-dynamic-range imaging (LDRI) 有更高的動態範圍(Dynamic range)，也就是在單一 Channel 中更精細的記錄影像內容。 Comparing with LDR image (Low-dynamic-range), HDR image (High-dynamic-range) has larger dynamic range, which means that HDR image can record image more precisely. 乍看之下「 HDR 影像格式比起 LDR 有更精細記錄影像的能力」似乎就是其影像更加鮮明的原因，不過這樣的想法存在著問題。
At first glance, “HDR image format has more precise recording ability than LDR image” seems to be the reason why HDR image looks so vivid. However, it contains logically defect.
查看 Wikipedia HDR 頁面後，會發現有更精細的記緣能力只說了一半的故事，另一半同等重要的是顯示，由於傳統顯示器無法完全顯示HDR影像的高對比度，HDR影像必需經過 Tone mapping ，把高動態範圍映射到較窄的動態範圍上。不過在同樣的傳統顯示器上，為何 HDR 影像經過 Tone mapping ，看起來還是比 LDR 影像來的鮮明？
After look up Wikipedia HDR pages, you will find that “more precise recording ability” is only half the story. Another equivalently important part is image display. Because traditional monitor cannot fully display high contrast of HDR image, HDR image has to be apply tone mapping before display. (Mapping higher dynamic range to lower dynamic range.) However, why did HDR image after tone mapping is still more vivid than LDR image in the same traditional monitor?
上述的疑問推翻了 HDR 影像只因為有更廣的記錄能力，而看起來較鮮明的說法(因為在顯示時，HDR、LDR影像都在同樣的窄對比值域上)。
The question above disprove that HDR image looks more vivid just because it has wider recording ability. (Because HDR and LDR image are on the same display dynamic range on monitor.)
其實 HDR 影像鮮明的秘密就藏在拍攝時！
Actually, the secret of vividly HDR image could be found if we observe what camera did when it take a shoot!
一般的攝影設備，如果想呈現影像暗部的細節，就必需增加曝光，但是反之，為了呈現暗部而增加曝光卻會犧牲亮部細節。會有這樣的兩難是源於攝影設備有其拍攝的動態範圍，例如 High-end DSLR camera (contrast 28500:1) 還是無法完全捕捉到和人眼動態時的動態範圍一樣廣(1000000:1)。
If we want to reveal detail in the darkest shadow, we have to adjust the exposure value(EV) higher when we are using a commercial camera. But in the meanwhile, the detail of brightest part will despair. These dilemma is causing by the dynamic range of a camera. For example, even a high-end DSLR camera (contrast 28500:1) cannot capture image with the same dynamic range of human eyes in dynamic (1000000:1).
為了重現如人眼般的動態範圍，可以透過結合多張 LDR 影像來合成一張 HDR 影像，也就是說因為在「記錄」影像時，不用特別的犧牲亮部或暗部，HDR 影像在拍攝時記錄了完整的動態範圍，而這就是 HDR 影像看起來如此鮮明的關鍵！
To reproduce eyes-like dynamic range, we could combine several LDR image to generate one HDR image. For this, we do not need to scarify detail of bright or dark part of image. HDR image record complete dynamic range when we take a shoot. This is why HDR image looks so vivid!
參考資料：
- High-dynamic-range imaging - Wikipedia - https://en.wikipedia.org/wiki/High-dynamic-range_imaging
- Dynamic range - Wikipedia - https://en.wikipedia.org/wiki/Dynamic_range#Photography
- Tone mapping - Wikipedia - https://en.wikipedia.org/wiki/Tone_mapping
本著作係採用創用 CC 姓名標示-非商業性-相同方式分享 4.0 國際 授權條款授權.
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