Abstract: A transaction document reader reads image data on a transaction document. The image data includes marking areas employed by a user to record marks and includes characters. A sensor senses, one row at a time, rows of image data as the document is transported across the sensor. Each row includes a number of pixel areas. The pixel areas of the rows being aligned in columns. The sensor generates output signals representative of the pixel areas. The output signals are transformed into bit signals representative of each pixel area. The bit signals are stored in image memory which is organized into rows and columns of bit signals corresponding to the rows and columns of the pixel areas of the image data. The marking areas are located in image memory. The marks are identified in the marking areas while additional rows of bit signals are being stored and while other marking areas are being located by probing the bit signals in image memory.