Image forming apparatus

1. An image forming apparatus comprises:

image-forming units of multiple colors that form images on an image carrier in such a manner that the images are overlaid on one another, and

a determination unit that determines necessity of position alignment correction of an image of each color accompanying image formation in accordance with a degree of change of image formation environment from when correction was executed in the past and a combination of colors used for the image formation.

2. The image forming apparatus according to claim 1 , wherein the position alignment correction is to align a position of an image of a color used for the image formation other than a reference color among the multiple colors with a position of an image of the reference color as a reference, and

the image forming apparatus comprises a storing unit that stores execution history of the position alignment correction of an image,

wherein when the determination unit determines that change of image formation environment from when the latest correction for the color was executed to the present moment is within a predetermined range for all colors used for the image formation other than the reference color among the colors on the basis of the history stored in the storing unit, the determination unit determines that the position alignment correction is unnecessary.

3. The image forming apparatus according to claim 2 , wherein the storing unit stores, as the history, a color for which the position alignment correction is performed and a parameter representing image formation environment during execution of correction in association with each other.

4. The image forming apparatus according to claim 1 comprising a storing unit that stores execution history of the position alignment correction of an image,

wherein the image forming apparatus has a plurality of modes that are different in a combination of colors used for image formation, and

when the determination unit determines, based on the history stored in the storing unit, that the latest position alignment correction is executed in a mode in which all colors used in a currently set mode are used and that change of image formation environment from when the latest correction was executed to the present moment is within a predetermined range, then the determination unit determines that the position alignment correction is unnecessary.

5. The image forming apparatus according to claim 4 , wherein the storing unit identifies, on the basis of a mode in which the position alignment correction is performed, all modes to which a parameter indicating image formation environment during execution of the correction in the mode can be applied, and stores the parameter indicating the image formation environment during execution of the correction in association with each mode to which the parameter can be applied.

6. The image forming apparatus according to claim 4 , wherein the storing unit stores, as the history, a parameter indicating image formation environment during execution of the correction in a common storage region regardless of a mode in which the position alignment correction is performed, and stores a flag indicating the mode in which the position alignment correction is performed last time.

7. The image forming apparatus according to claim 2 comprising an identifying unit that identifies, from among colors used for the image formation other than the reference color, a color for which change of image formation environment from when the latest correction for the color was executed to the present moment is out of a predetermined range, on the basis of the history stored in the storing unit,

wherein the position alignment correction is performed for a color identified by the identifying unit from among colors used for image formation other than the reference color.

8. An image forming method comprises:

forming images of multiple colors on an image carrier in such a manner that the images are overlaid on one another, and

determining necessity of position alignment correction of an image of each color accompanying image formation in accordance with a degree of change of image formation environment from when correction was executed in the past and a combination of colors used for the image formation.

9. An image forming apparatus comprises:

means for forming images of multiple colors on an image carrier in such a manner that the images are overlaid on one another, and a means for determining necessity of position alignment correction of an image of each color accompanying image formation in accordance with a degree of change of image formation environment from when correction was executed in the past and a combination of colors used for the image formation.