16. A method for providing an adaptive video transcoding framework for a video hosting service, the method comprising: estimating video coding complexity of a source video, the source video having a plurality of coding parameters and a first resolution; training one or more rate-distortion models and scaling models from a plurality of different encoded videos of a video corpus; transcoding the source video based on the estimated video coding complexity of the source video and a target bitrate estimated by rate-distortion models and scaling models trained by a video rate-distortion modeling engine; and determining an optimal resolution for the source video based on the estimated video coding complexity of the source video, and to transcode the source video with the first resolution into an output video with the optimal resolution.
16. A method for providing an adaptive video transcoding framework for a video hosting service, the method comprising: estimating video coding complexity of a source video, the source video having a plurality of coding parameters and a first resolution; training one or more rate-distortion models and scaling models from a plurality of different encoded videos of a video corpus; transcoding the source video based on the estimated video coding complexity of the source video and a target bitrate estimated by rate-distortion models and scaling models trained by a video rate-distortion modeling engine; and determining an optimal resolution for the source video based on the estimated video coding complexity of the source video, and to transcode the source video with the first resolution into an output video with the optimal resolution. 17. The method of claim 16 , wherein estimating video coding complexity of a source video comprises: training a video coding complexity model using a plurality of videos selected from among the plurality of different encoded videos of the video corpus; receiving the source video, the source video having a first video format; encoding the source video into an intermediate video format; extracting video features from the encoded source video; and computing a video coding complexity score based on the extracted video features and the trained video coding complexity model.