1. Field of the Invention
The present invention relates to an image forming apparatus having a scanning unit which scans a light beam on a photosensitive member in accordance with image data.
2. Description of the Related Art
Recent electrophotographic image forming apparatuses form images by reflecting a laser beam emitted in accordance with image data by a rotating polygonal mirror, and scanning the laser beam on a photosensitive member. When a rotating shaft 283c of a polygonal mirror 283b inclines from an ideal position (FIG. 3A) by an angle θ, a laser beam fluctuates in the circumferential direction of a photosensitive member 21 while scanned by one line in the main scanning direction, as shown in FIG. 3B. If the laser beam fluctuates in the circumferential direction of the photosensitive member 21, the scan path curves as represented by a solid line 402 in FIG. 4 while the laser beam is scanned by one line in the main scanning direction. In a color image forming apparatus, the scan path of a laser beam sometimes differs between respective colors. Image misregistration between the colors occurs and appears as a blurred (unsharp) image.
To suppress the fluctuations in the circumferential direction of the photosensitive member, correction lenses having different refractive indices in the circumferential direction may be interposed in the optical path of a laser beam in accordance with scan positions. However, this method requires labor costs in processing, attachment, and adjustment of correction lenses, raising the cost.
To prevent this, Japanese Patent Laid-Open Nos. 02-050176, 2003-276235, and 2005-304011 propose processing of making a curved scan path come close to an ideal one by scanning areas a and e by a scan line 502, areas b and d by a scan line 503, and an area c by a scan line 504, as shown in FIG. 5.
When this processing is executed, a jaggy sometimes stands out at a position where the scan line switches, as shown in FIG. 7A. Japanese Patent Laid-Open No. 2003-182146 proposes interpolation processing of smoothing the jaggy by PWM (Pulse Width Modulation)-controlling a laser beam before and after switching the scan line, as shown in FIG. 7B.
Images to be formed by an image forming apparatus include texts, line images, and halftone images such as a photograph. The image forming apparatus forms an image by executing dither processing or the like for halftone image data.
However, if image data having undergone dither processing undergoes interpolation for smoothing, the density in the dither pattern is unbalanced, or small dots are generated, as shown in FIG. 11C. The unbalanced density in the dither pattern and an unstable toner amount attached to small dots influence the reproducibility of the halftone density.