为什么较低的qmax值会改善视频质量？ [英] Why low qmax value improve video quality?
问题描述
也许我的问题由于不了解而没有意义，但请向我解释我错过了什么，因为我确实阅读了帖子和Wiki，但对我来说仍然不清楚。
Maybe my questions doesn't make sense due to not understanding but please explain me what I miss because I did read posts and wiki and still it's not clear to me.
据我了解，为qmax设置较低的值将通过增加比特率来改善质量。
也许我听不懂但不降低Q（量化）会降低量化水平，从而降低比特率，这意味着质量下降？还是在ffmpeg中降低Q意味着增加量化水平？如果最后一个为真，则较低的qmax可以改善质量。
As I understand setting low value for qmax will improve the quality by increasing the bitrate. Maybe I didn't understood something but isn't lowing the Q(quantization) will decrease the quantization levels and thus the bitrate which means degradation in quality? Or in ffmpeg lowing Q means increasing the quantization levels? If the last is true so it make sense that lower qmax improves the quality.
如果上述为真，则增大qmax会降低量化级别，这意味着编码量化级别。因此，如果某个级别的位数较少，则每帧的总位数会减少，那么编码器如何设法达到所需的比特率？
If the above is true, so increasing qmax will decrease the quantization levels which means less bits for coding a quantization level. So, if number of bits for a level is lower, so total bits per frame will be lower, so how the encoder manage to get to the desired bitrate?
推荐答案
您正确理解量化因子和比特率之间的关系。
Your are right in your interpretation of the relation between quantization factor and bit-rate.
- 最低量化越高，比特率和质量就越高。
- 量化越高，比特率和质量就越低。
但是无论如何，对于给定的量化器，您仍然可以要求目标比特率，如果有，您有2种情况：
But in any case, for a given quantizer, you can still ask for a target bit rate, and if so you have 2 cases:
- 由于比特率高于目标值ans，因此速率控制过程将使用更高的量化级别。
但是对于ffmpeg，qmax可能有不同的含义，因为它是编解码器相关的参数。
for x264，它应该是一个量化器，请参见此处，
但是对于其他一些编解码器，它不是量化级别而是质量范围。
But with ffmpeg qmax may have different meaning as it's a codec dependent parameters. for x264 it should be a quantizer see here, But with some others codecs it doesn't represent a quantization level but a quality range.
qmax和qmin是质量范围 ，您可以在其中定义编码。
相反，我至少会期望
的值越高，质量越低。
qmax and qmin are the 'quality-ranges' in which you define to encode. Oposite from what most, atleast me, would expect is that higher the values the lower the quality.
- qmin 50和qmax 51给出最低质量
- qmin 0 -qmax 1给出最高质量
小于16的qmin值和最大qmax的26值显然是非常好。
将qmin减小到16以下会占用额外的空间，而不会增加可见的质量。
values of qmin lower than 16 and qmax 26 are visibly 'very good' lowering qmin below 16 costs extra space 'without adding visible' quality.
因此，如果您提高视频质量，则编码后的输出将更接近原始视频，并且通常需要更高的比特率，但是在内部它通常意味着使用较低的量化级别。
so if you increase the video quality the encoded output will be closer to the original, and that often require a higher bit-rate, but internally it often means that lower quantization level are use.
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