When printing photo images taken with a digital camera, the images go through various processes from shooting to the printing phase. In these processes, noise and tone jump (luminance value jump) occur on the images due to the characteristics of each process, such as the characteristics of image sensors, signal amplification, analog-to-digital conversion, color conversion, and sharpness adjustment. Noise in the low-frequency domain appears as surface roughness; tone jump appears as gradation discontinuity in printed images. Such noise and tone jump lead to image degradation.
Fig. 1: Image with Reduced Noise and its Power Spectrum Transition
Fig. 2: Image with Reduced Tone Jump and its Luminance Value Transition
To address these problems, Fuji Xerox has developed noise-controlling technology for digital images. This technology reduces noise presented as roughness in the low-frequency domain, while increasing noise in the high-frequency domain to control and prevent tone jump that causes gradation discontinuity. Fig. 1 shows images with and without the noise-controlling process, and a graph that plots power spectrums (noise strength) against spatial frequency after transforming luminance components inside the squared area by Fourier Transform. The plotted green line in Fig. 1 indicates reduced roughness in the image thanks to the noise-controlling process, because noise in the low-frequency domain is reduced, while noise in the high-frequency domain is increased. Fig. 2 shows images with and without the noise-controlling process in the gradation area, and a graph that plots the luminance components along with a diagonal line drawn in the images. In the image subject to the noise-controlling process, the luminance value jump that represents tone jump is alleviated (as indicated by the green line in the graph of Fig. 2), while the process also smoothens gradation.