1. An image forming apparatus comprising: an image information obtainer to obtain image information; a gradation sequence processor to apply at least two different types of gradation sequence processing to the image information; a toner image formation device to form a toner image on a recording medium based on the image information; a fixing device to fix the toner image onto the recording medium in a fixing process; a controller including a start-up temperature changer to change a start-up temperature of the fixing device during a start-up mode of the fixing device before the fixing process based on whether a halftone process is applied in the image information and a type of gradation sequence processing applied to the image information when the halftone process is applied; and a region detector to detect inclusion of one of character and photographic regions on an image, wherein if the halftone process is applied and the gradation sequence processor applies an error diffusion method, then the controller selects a first start-up temperature, wherein if the halftone process is applied and the gradation sequence processor applies a dither method, then the controller selects the first or a second start-up temperature, wherein if the halftone process is not applied, then the controller selects a third start-up temperature, wherein one of said at least two different types of gradation sequence processing includes at least two different types of dither method, wherein a type of a dither method applied and a respective number of lines employed by the dither method applied are changed in accordance with a detection result obtained by the region detector, wherein said start-up temperature is selected in accordance with the type of dither method applied to the image information and the respective number of lines employed by the dither method applied when one of the at least two different types of dither method is employed, wherein if a character region is included, halftone processing is applied and a speed priority general document mode is selected, then the controller selects the second start-up temperature, wherein if the character region is included, halftone processing is applied and the speed priority general document mode is not selected, then the controller selects the first start-up temperature, wherein if the character region is included, and halftone processing is not applied, but a photograph region is included, then the controller selects the third start-up temperature, wherein if the character region is included, and halftone processing is not applied and the photograph region is not included, then the controller selects the third start-up temperature, wherein if the character region is not included but the photograph region is included, and halftone processing is applied, then the controller selects the second start-up temperature, and wherein if the character region is not included, and either the photograph is region is not included or halftone processing is not applied, then the controller selects the third start-up temperature.