An integer tone mapping operation for HDR images in OpenEXR with denormalized numbers

We propose an integer tone mapping operator (TMO) for high dynamic range (HDR) images expressed in a floating-point data format. Two purposes are achieved by the proposed TMO. The first purpose is to implement a TMO with less memory space. The second purpose is to give an important step to realize a fixed-point TMO. The proposed TMO is available for HDR images in the OpenEXR format. The OpenEXR format has two numerical representations (the normalized number and the denormalized number) which are not in other HDR formats such as RGBE. These two numerical representations cause a problem in applying an integer TMO. The proposed method enables us to avoid the problem by using the intermediate format. Moreover, the exponent part and the mantissa part are processed separately as two integer numbers. As a result, an integer TMO with less numerical range is achieved by our method. The experimental results show that the proposed method can generate high-quality low dynamic range (LDR) images with less memory space.

1. Introduction

Recently, high dynamic range (HDR) images have been spreading rapidly from the fields of photography and computer graphics, to the other fields such as medical imaging and car-mounted camera. On the contrary, the next generation display devices which can accept so wide range of dynamics of pixel values in HDR images are not popular yet. There-fore, a tone mapping operation (TMO) is important to reduce dynamic range of HDR images so that is can be treated with conventional display devices.