The hybrid Stereolithography (SLA) process integrates the laser scanning based SLA system and the mask projection based SLA system. Multiple laser paths are used to scan the border of a 2D pattern, whereas a mask image is adopted to solidify the interior area. By integrating the merits of two subsystems, the hybrid SLA process can achieve high surface quality without sacrificing the productivity. For the hybrid system, closed polygonal contours are required to direct the laser scanning, and a binary image is also needed for the mask projection. We proposed a novel image-based slicing method. This approach can convert a 3D model into a series of binary images directly, and each image is corresponding to the cross-section of the model at a specific height. Based on the resultant binary image, we use the image processing method to gradually shrink the pattern in the image. The boundaries of the shrunk image are traced and then restored as polygons to direct the laser spot movement. The final shrunk image will serve as the input for the mask projection. The experimental results of several test cases demonstrate that the proposed method is substantially more efficient than the traditional approach. Its accuracy is also studied and discussed.