Image generation apparatus, image generation method, image generation program, and integrated circuit for rendering a target pixel in a target scene by using Z-buffering

1. An image generation apparatus that renders a target pixel in a target scene by using Z-buffering, comprising:

a processor; and

a non-transitory computer-readable medium having stored thereon executable instructions that, when executed by said processor, cause said image generation apparatus to function as:

a statistical value calculation unit that calculates a statistical value, the statistical value indicating characteristics of a distribution of Z depth values in a predetermined scene, the Z depth values corresponding one-to-one to pixels in the predetermined scene and each indicating a depth of a corresponding pixel in the predetermined scene;

a Z depth value conversion unit that, by using the statistical value, converts a Z depth value for the target pixel so as to obtain a converted Z depth value;

a Z depth value generation unit that generates a first Z depth value, having fewer bits than the Z depth value for the target pixel, for the target pixel by using the converted Z depth value and generates a second Z depth value for the target pixel by using either one of the converted Z depth value and the Z depth value for the target pixel; and

a comparison unit that (i) performs a first comparison of depths by using the first Z depth value for the target pixel as one subject of comparison and, (ii) only when the first comparison cannot be successfully performed, performs a second comparison of the depths by using the second Z depth value for the target pixel as one subject of comparison, the first Z depth value to be stored to a first memory and the second Z depth value to be stored to a second memory.

2. The image generation apparatus of claim 1 , wherein

the statistical value includes an average value of the Z depth values in the predetermined scene and a standard deviation of the Z depth values in the predetermined scene, and

the converted Z depth value for the target pixel is a standard score, in the predetermined scene, of the Z depth value for the target pixel.

3. The image generation apparatus of claim 2 , wherein

the statistical value calculation unit includes a plurality of counters holding values indicating a frequency distribution of the Z depth values in the predetermined scene, and

the statistical value calculation unit calculates the average value and the standard deviation by using the values held by the counters.

4. The image generation apparatus of claim 1 , wherein

the statistical value includes a maximum Z depth value and a minimum Z depth value among the Z depth values in the predetermined scene, and

the conversion of the Z depth value for the target pixel by the Z depth value conversion unit is performed such that a converted Z depth value for the maximum Z depth value in the predetermined scene corresponds to a maximum expressible value of the converted Z depth value and a converted Z depth value for the minimum Z depth value in the predetermined scene corresponds to a minimum expressible value of the converted Z depth value.

5. The image generation apparatus of claim 1 , wherein

the predetermined scene is a scene immediately preceding the target scene.

6. The image generation apparatus of claim 1 , wherein

the predetermined scene is the target scene, rendering of which is being performed.

7. The image generation apparatus of claim 1 , wherein

an internal representation of the converted Z depth value is in a floating-point format.

8. The image generation apparatus of claim 1 , wherein

an internal representation of the converted Z depth value is in a fixed-point format.

9. The image generation apparatus of claim 1 , wherein

the first Z depth value is a value represented by high order bits of the converted Z depth value, and

the second Z depth value is a value represented by low order bits of the converted Z depth value.

10. The image generation apparatus of claim 1 , wherein

the first Z depth value is a value represented by all bits of the converted Z depth value, and

the second Z depth value is the Z depth value for the target pixel.

11. The image generation apparatus of claim 1 , wherein

the first memory operates at a higher speed than the second memory.

12. The image generation apparatus of claim 1 , wherein

the first memory operates while consuming less electrical power than the second memory.

13. The image generation apparatus of claim 1 , wherein

the statistical value calculation unit divides a screen region onto which the predetermined scene is rendered into a plurality of rendering regions and calculates, as the statistical value, a statistical value of each of the rendering regions.

14. The image generation apparatus of claim 1 , wherein

the statistical value calculation unit divides the distribution of Z depth values in the predetermined scene into a plurality of partial distributions and calculates, as the statistical value, a statistical value of each of the partial distributions,

the Z depth value conversion unit converts the Z depth value for the target pixel into the converted Z depth value by using, as the statistical value, a statistical value for a corresponding partial distribution, the corresponding partial distribution being specified according to the Z depth value for the target pixel,

the Z depth value generation unit generates, in addition to the first Z depth value and the second Z depth value, a third Z depth value that identifies the corresponding partial distribution, and

the comparison unit performs a third comparison of the depths by using the third Z depth value as one subject of comparison, and performs the first comparison only when the third comparison cannot be successfully performed, the third Z depth value to be stored to a third memory.

15. The image generation apparatus of claim 14 , wherein

the third memory operates at a higher speed than the first memory.

16. The image generation apparatus of claim 14 , wherein

the third memory operates while consuming less electrical power than the first memory.

17. An image generation method for rendering a target pixel in a target scene by using Z-buffering, comprising:

calculating a statistical value, the statistical value indicating characteristics of a distribution of Z depth values in a predetermined scene, the Z depth values corresponding one-to-one to pixels in the predetermined scene and each indicating a depth of a corresponding pixel in the predetermined scene;

converting, by using the statistical value, a Z depth value for the target pixel so as to obtain a converted Z depth value;

generating a first Z depth value, having fewer bits than the Z depth value for the target pixel, for the target pixel by using the converted Z depth value and generating a second Z depth value for the target pixel by using either one of the converted Z depth value and the Z depth value for the target pixel; and

performing a first comparison of depths by using the first Z depth value for the target pixel as one subject of comparison and, only when the first comparison cannot be successfully performed, performing a second comparison of the depths by using the second Z depth value for the target pixel as one subject of comparison, the first Z depth value to be stored to a first memory and the second Z depth value to be stored to a second memory.

18. A non-transitory computer-readable medium having a program stored thereon, the program including instructions for causing a computer to execute an image generation method of rendering a target pixel in a target scene by using Z-buffering, the image generation method comprising:

calculating a statistical value, the statistical value indicating characteristics of a distribution of Z depth values in a predetermined scene, the Z depth values corresponding one-to-one to pixels in the predetermined scene and each indicating a depth of a corresponding pixel in the predetermined scene;

converting, by using the statistical value, a Z depth value for the target pixel so as to obtain a converted Z depth value;

generating a first Z depth value, having fewer bits than the Z depth value for the target pixel, for the target pixel by using the converted Z depth value and generating a second Z depth value for the target pixel by using either one of the converted Z depth value and the Z depth value for the target pixel; and

performing a first comparison of depths by using the first Z depth value for the target pixel as one subject of comparison and, only when the first comparison cannot be successfully performed, performing a second comparison of the depths by using the second Z depth value for the target pixel as one subject of comparison, the first Z depth value to be stored to a first memory and the second Z depth value to be stored to a second memory.

19. An integrated circuit for use in an image generation apparatus that renders a target pixel in a target scene by using Z-buffering, comprising:

a statistical value calculation circuit that calculates a statistical value, the statistical value indicating characteristics of a distribution of Z depth values in a predetermined scene, the Z depth values corresponding one-to-one to pixels in the predetermined scene and each indicating a depth of a corresponding pixel in the predetermined scene;

a Z depth value conversion circuit that, by using the statistical value, converts a Z depth value for the target pixel so as to obtain a converted Z depth value;

a Z depth value generation circuit that generates a first Z depth value, having fewer bits than the Z depth value for the target pixel, for the target pixel by using the converted Z depth value and generates a second Z depth value for the target pixel by using either one of the converted Z depth value and the Z depth value for the target pixel; and

a comparison circuit that (i) performs a first comparison of depths by using the first Z depth value for the target pixel as one subject of comparison and, (ii) only when the first comparison cannot be successfully performed, performs a second comparison of the depths by using the second Z depth value for the target pixel as one subject of comparison, the first Z depth value to be stored to a first memory and the second Z depth value to be stored to a second memory.