Abstract
Purpose: :
To implement a portable low vision aid using single camera for image capture and with a speed of real time image enhancement processing that can display on a head mount device (HMD) forming stereo vision.
Methods: :
One of the advantages of field-programmable gate array (FPGA), which is a semiconductor device that can be configured by the customer or designer after manufacturing, is that the calculations by software can be replaced by hardware and speeds up the image processing time. Contrast sensitivity functions (CSF) of low vision subjects (n=10, VA<0.1) were measured and visual degradation transfer function (VDTF) were obtained while compared to normals (n=30). The subject's spatial frequency domain was divided into 5 bands according to VDTF=0.5, 0.366, 0.233 and 0.1. Gaussian filter with different sizes was applied to the input image which captured by a ccd camera (640x480 pixels). Gains for enhancement in mid-freq bands were preset with values of 2-10; 1 for Hi-freq and nonlinear compression for Low-freq bands. Enhanced images were sent to a pair of HMD (800x600 pixels). Post processed images were rescaled or expanded ( from 640x480 to 800x600). The two images simultaneously sent to the pair of HMD were mutually shifted with some pixels and rotated with an angle (<10 deg) to gain a stereo vision for the subject. Parameters of shifting and rotation can be controlled by pressing interfacing buttons. A paragraphy of literature reading and a section of silent-movie watching, w/o enhancement, were carried out to validate subject's visual performance. Scoring the 'static' literature reading speed and 'dynamic' movie watch were according to the recording time and questionaire respectively.
Results: :
The speed of the image processing can be up to 64 frames/sec through the FPGA technique. Stereo image is demostrated. While compared to without doing image enhancement, the average reading speed is increased about 1.5-fold and the average score for silent-movie watching is also gained about 1.5-fold or higher after enhancement.
Conclusions: :
Through the employment of hi-tech and high-speed electronic components, the vision aid which requires fast image processing rate can be implemented. The technique can not be limited for low vision aid only, it can also be used in any area or devices where only one video signal input is available.
Keywords: aberrations • reading • visual impairment: neuro-ophthalmological disease