ALPHA-TO-COVERAGE VALUE DETERMINATION USING VIRTUAL SAMPLES

• US 20110090251A1
• Filed: 10/14/2010
• Published: 04/21/2011
• Est. Priority Date: 10/15/2009
• Status: Active Grant

• Current Assignee: NVIDIA Corporation
• Original Assignee: NVIDIA Corporation
• Inventors: Ohannessian, Robert, Amsinck, Christian, Molnar, Steven E., Kilgariff, Emmett M., Donovan, Walter E.
• Granted Patent: US 8,669,999 B2
• Time in Patent Office: Days
• US Class Current: 345/632
• CPC Class Codes: G06T 15/005 General purpose rendering architectures
• CPC Class Codes: G06T 15/503 Blending, e.g. for anti-aliasing
• CPC Class Codes: G09G 2360/123 using interleaving
• CPC Class Codes: G09G 5/363 Graphics controllers
• CPC Class Codes: G09G 5/393 Arrangements for updating the contents of the bit-mapped memory

Abstract

One embodiment of the present invention sets forth a technique for converting alpha values into pixel coverage masks. Geometric coverage is sampled at a number of “real” sample positions within each pixel. Color and depth values are computed for each of these real samples. Fragment alpha values are used to determine an alpha coverage mask for the real samples and additional “virtual” samples, in which the number of bits set in the mask bits is proportional to the alpha value. An alpha-to-coverage mode uses the virtual samples to increase the number of transparency levels for each pixel compared with using only real samples. The alpha-to-coverage mode may be used in conjunction with virtual coverage anti-aliasing to provide higher-quality transparency for rendering anti-aliased images.

Claims

1. A method of producing a pixel coverage mask based on an alpha value, the method comprising:

combining a virtual coverage mask that indicates which virtual samples and real samples within a pixel are covered by a graphics primitive and a quantized alpha mask based on the alpha value to produce the pixel coverage mask;

determining a link field that indicates links between the virtual samples and the real samples;

adjusting the link field based on the pixel coverage mask to link any orphaned virtual sample to one of the real samples, wherein each virtual sample inherits color data computed for the real sample to which the virtual sample is linked; and

outputting the pixel coverage mask and the link field for the pixel.

2. The method of claim 1, further comprising the step of determining the quantized alpha mask based on a sampling mode that defines a first number of the real samples and a second number of the virtual samples in the pixel.

3. The method of claim 2, wherein the sampling mode defines affinities that control the links between the virtual samples and the real samples.

4. The method of claim 2, further comprising the steps of:

determining an alpha-to-coverage ordering of the real samples and virtual samples based on the sampling mode; and

enabling bits of the quantized alpha mask based on the alpha-to-coverage order.

5. The method of claim 4, wherein at least one of the bits in the quantized alpha mask corresponding to the real samples is enabled before any of the bits in the quantized alpha mask corresponding to the virtual samples are enabled.

6. The method of claim 1, wherein each one of the virtual samples may link to any one of the real samples.

7. The method of claim 1, wherein each one of the virtual samples may link to a subset of the real samples.

8. The method of claim 1, wherein the step of adjusting the link field comprises linking an orphaned virtual sample to at least one real sample that is masked by the pixel coverage mask.

9. The method of claim 1, further comprising the steps of:

computing a weight for the real samples that are enabled according to the pixel coverage mask, wherein the weight is increased for each virtual sample that links to one of the real samples; and

scaling the color data for the real samples that are enabled according to the pixel coverage mask by the computed weights to produce final color data for the real samples.

10. A computer-readable storage medium storing instructions that, when executed by a processor, cause the processor to produce a pixel coverage mask based on an alpha value, by performing the steps of:

combining a virtual coverage mask that indicates which virtual samples and real samples within a pixel are covered by a graphics primitive and a quantized alpha mask based on the alpha value to produce the pixel coverage mask;

determining a link field that indicates links between the virtual samples and the real samples;

adjusting the link field based on the pixel coverage mask to link any orphaned virtual sample to one of the real samples, wherein each virtual sample inherits color data computed for the real sample to which the virtual sample is linked; and

outputting the pixel coverage mask and the link field for the pixel.

11. A system for producing a pixel coverage mask based on an alpha value, the system comprising:

a processor including an alpha-to-coverage unit that is configured to;

combine a virtual coverage mask that indicates which virtual samples and real samples within a pixel are covered by a graphics primitive and a quantized alpha mask based on the alpha value to produce the pixel coverage mask;

determine a link field that indicates links between the virtual samples and the real samples;

adjust the link field based on the pixel coverage mask to link any orphaned virtual sample to one of the real samples, wherein each virtual sample inherits color data computed for the real sample to which the virtual sample is linked; and

output the pixel coverage mask and the link field for the pixel.

12. The system of claim 11, further comprising a memory storing instructions that, when executed by the processor, configure the alpha-to-coverage unit to:

combine the virtual coverage mask and the quantized alpha mask;

determine the link field;

adjust the link field; and

output the pixel coverage mask and the link field for the pixel.

13. The system of claim 11, wherein the alpha-to-coverage unit is further configured to determine the quantized alpha mask based on a sampling mode that defines a first number of the real samples and a second number of the virtual samples in the pixel.

14. The system of claim 13, wherein the sampling mode defines affinities that control the links between the virtual samples and the real samples.

15. The system of claim 13, wherein the alpha-to-coverage unit is further configured to:

determine an alpha-to-coverage ordering of the real samples and virtual samples based on the sampling mode; and

enable bits of the quantized alpha mask based on the alpha-to-coverage order

16. The system of claim 15, wherein at least one of the bits in the quantized alpha mask corresponding to the real samples is enabled before any of the bits in the quantized alpha mask corresponding to the virtual samples are enabled.

17. The system of claim 11, wherein each one of the virtual samples may link to any one of the real samples.

18. The system of claim 11, wherein each one of the virtual samples may link to a subset of the real samples.

19. The system of claim 11, wherein the adjusting of the link field comprises linking an orphaned virtual sample to at least one real sample that is masked by the pixel coverage mask.

20. The system of claim 11, wherein the processor is further configured to:

compute a weight for the real samples that are enabled according to the pixel coverage mask, wherein the weight is increased for each virtual sample that links to one of the real samples; and

scale the color data for the real samples that are enabled according to the pixel coverage mask by the computed weights to produce final color data for the real samples.