| 摘要 |
<p>PURPOSE:To keep the approximately constant quality of a compressed image by changing the value of a scale factor according to the contents of a picture to be compressed. CONSTITUTION:When the image data is compressed, the scale factor is changed at quantization in accordance with the contents of an image to be compressed are coincident with a fine part of a pattern where the image signal has a sudden change or a flat part of the pattern where the image signal has a slow change. A fact whether the contents of the image are coincident with the fine or flat part of the pattern is decided by detecting the DCT spectrum of each of those blocks forming a sheet of image. Thus an area ratio is obtained between the fine and flat parts of the pattern in a sheet of image. Then a scale factor is obtained in accordance with an area ratio obtained based on the relation between the area ratio obtained previously via a visual test and the scale factor. The scale factor thus obtained is multiplied by each threshold value of a quantized matrix for the execution of the quantization.</p> |
