Printer and printing system

1. A printer comprising:

a first printing unit responsible for one of preceding printing and subsequent printing that are to be performed on one sheet in overprinting, an other of the preceding printing and the subsequent printing being performed by a second printing unit; and

a base-printing controller that causes the first printing unit to print a base for printing by the second printing unit in an operation of forming a test pattern for registration between the preceding printing and the subsequent printing on the sheet.

2. The printer according to claim 1 ,

wherein the test pattern is a combination of a first pattern to be printed by the first printing unit and a second pattern to be printed by the second printing unit, and

wherein the base-printing controller causes the first printing unit to print the base such that the base appears at an edge of a mark to be printed over the base by the second printing unit, the mark being included in the second pattern.

3. The printer according to claim 2 ,

wherein the base-printing controller causes the first printing unit to print the base with a colorant that produces, when the test pattern is read, a greater signal difference between the mark printed over the base and the base appearing at the edge of the mark than a signal difference between a portion of the sheet where no base is present and the mark on the portion of the sheet.

4. The printer according to claim 2 ,

wherein one of the base and the mark is printed with a white colorant, and an other of the base and the mark is printed with a black colorant.

5. The printer according to claim 2 ,

wherein the colorant for the base has a color complementary to a color of a colorant for the mark.

6. The printer according to claim 2 ,

wherein the first printing unit is operable with colorants having a plurality of colors, respectively, and

wherein a colorant for the base is one of the colorants handleable with the first printing unit that has a greatest hue difference from a colorant for the mark.

7. The printer according to claim 2 ,

wherein the first printing unit is operable with colorants having a plurality of colors, respectively, and

wherein a colorant for the base is one of the colorants handleable with the first printing unit that has a greatest brightness difference from a colorant for the mark.

8. The printer according to claim 2 ,

wherein the base-printing controller causes the first printing unit to print the base such that the base appears around an entire edge of the mark.

9. The printer according to claim 2 ,

wherein the base-printing controller causes the first printing unit to print the base such that the base appears on two sides of the mark in a predetermined first direction.

10. The printer according to claim 9 ,

wherein the first direction is a sheet-transporting direction in printing.

11. The printer according to claim 9 ,

wherein the first direction is a sheet-width direction intersecting a sheet-transporting direction in printing.

12. The printer according to claim 2 ,

wherein the mark includes a plurality of marks that are each elongated in a second direction intersecting a predetermined first direction and are arranged side by side in the first direction, and

wherein the base-printing controller causes the first printing unit to print the base such that the base appears on two sides of each of the marks in the first direction.

13. The printer according to claim 2 ,

wherein the mark has a shape with a corner, and

wherein the base-printing controller causes the first printing unit to print the base such that the base appears at the corner of the mark.

14. The printer according to claim 2 ,

wherein the mark includes two line segments extending in two respective directions in such a manner as to form an intersection, and

wherein the base-printing controller causes the first printing unit to print the base such that the base appears around the intersection.

15. The printer according to claim 2 ,

wherein the mark includes two marks that are spaced apart from each other, and

wherein the base-printing controller causes the first printing unit to print the base such that the base appears in such a manner as to fill the space between the two marks.

16. A printing system comprising:

a first printing unit and a second printing unit that are each responsible for a corresponding one of preceding printing and subsequent printing that are to be performed on one sheet in overprinting; and

a base-printing controller that causes the first printing unit to print a base for printing by the second printing unit in an operation of forming a test pattern for registration between the preceding printing and the subsequent printing on the sheet.

17. The printing system according to claim 16 ,

wherein the base-printing controller allows the first printing unit to print the base or prohibits the first printing unit from printing the base, in accordance with a colorant for the mark.

18. The printing system according to claim 17 ,

wherein the base-printing controller allows the first printing unit to print the base or prohibits the first printing unit from printing the base, in accordance with a colorant for the mark and a color of the sheet.

19. The printing system according to claim 18 ,

wherein the base-printing controller causes the first printing unit to print the base with a colorant determined in accordance with a colorant for the mark and a color of the sheet.

20. The printing system according to claim 16 ,

wherein the test pattern is a combination of a first pattern to be printed by the first printing unit and a second pattern to be printed by the second printing unit, the second pattern including a mark that is printed over the base, and

wherein the base-printing controller causes the first printing unit to print the base with a colorant determined in accordance with a colorant for the mark.