视效制作中镜头畸变流程的优化设计

A research on optimisation design of lens distortion workflow in visual effects

在视效制作过程中,为了获得更加真实和无痕的视觉效果,数字艺术家在将三维渲染或数字绘景等元素合成匹配到实拍镜头画面之前首先需要进行运动匹配处理,以使最终合成的画面看起来犹如是从同一摄像机、同一场景“拍摄”而成。在运动匹配环节,跟踪师首先需要对实拍镜头画面进行去畸变处理以获得去畸变镜头。但是,镜头畸变的处理不可避免会引起去畸变镜头画面尺寸有别于原始实拍镜头画面尺寸的情况。为了解决镜头畸变处理过程中画面尺寸的缩放给跟踪、动画、特效、灯光渲染以及合成等下游环节带来的诸多不便与麻烦,本文以数字合成为切入点,提出基于影视全流程视野进行镜头畸变的优化设计方案,即使用表达式语句精准、自动获得去畸变镜头画面的缩放参数值和缩放中心点位置,并且实现在不改变画面尺寸的情况下进行镜头畸变的正向去除与反向添加。该优化设计方案不仅使得视效全流程都能够基于相同的画面尺寸开展视效设计与制作,而且将有效解决因镜头画面尺寸不统一而带来的诸多不便与错误。

In post-production,in order to obtain a more realistic and seamless visual effect,digital artists need to base on Matchmoving to integrate CGI and matte painting to original plate,and make the final comp shot looks as if it is shoted from the same camera and the same scene.In the process of Matchmoving,the tracker artist first needs to undistort the original plate to obtain the undistortion plate.However,the processing of lens distortion will inevitably cause the image size of the undistortion plate to be different from that of the original plate.In order to solve the problem of image size scaled during lens distortion processing,it brings many inconveniences and trouble to the downstream of the VFX pipeline such as Matchmoving,animation,simulation,lighting and compositing,etc.Based on the digital compositing production process,this paper proposes an optimized design of lens distortion based on the VFX pipeline,That is,based on expressions automatically obtain the zoom parameter value and center position value of the undistortion plate,and during removal or addition of lens distortion without changing the image size.The optimized design not only enables the whole visual effect process based on the same image size,but also effectively solves many inconveniences and errors caused by the non-uniform image size.