GPU rendering of knockout groups

1. A computer-implemented method of rendering knockout groups, the method comprising:

compositing, by a graphics processing unit (GPU), each object of a knockout group with a backdrop of the knockout group, the knockout group including a plurality of objects encoded in a document, using

means for blending a color of each pixel representing the respective object with a color of a pixel in the backdrop of the knockout group overlapped by the respective pixel independently of a shape of any of the objects of the knockout group, thereby producing an intermediate color of the respective pixel, and

means for computing a final color of each pixel representing the respective object as a function of the intermediate color, the color of the pixel in the backdrop prior to the blending, and a fractional coverage value of the shape of the respective object, thereby knocking out at least a portion of the pixel in the backdrop based on the shape of the respective object, the final color of each pixel being computed by adding a product of the fractional coverage value and the intermediate color to a product of a complement of the fractional coverage value and the color of the pixel in the backdrop;

storing the final color of each pixel in a framebuffer of the GPU for rendering on a display device; and

generating, using the GPU, a rendered image based on the final color of each pixel stored in the framebuffer.

2. The method of claim 1 , further comprising generating, by the GPU, data representing a single-sampled shape texture of the knockout group.

3. The method of claim 2 , further comprising attaching, by the GPU, the single-sampled shape texture of the knockout group to the framebuffer.

4. The method of claim 2 , further comprising recording, by the GPU, an accumulated shape of all of the objects of the knockout group in the single-sampled shape texture of the knockout group.

5. The method of claim 2 , further comprising consolidating, by the GPU, the single-sampled shape texture of the knockout group with a single-sampled shape texture of a parent knockout group including a different plurality of objects encoded in the document.

6. The method of claim 5 , further comprising compositing each object of the parent knockout group with a backdrop of the parent knockout group, wherein the backdrop of the parent knockout group comprises the composited objects and consolidated shapes of the knockout group.

7. A system for rendering knockout groups, the system comprising:

a storage; and

a graphics processing unit (GPU) operatively coupled to the storage, the GPU configured to execute instructions stored in the storage that when executed cause the GPU to carry out a process including

compositing, by the GPU, each object of a knockout group with a backdrop of the knockout group, the knockout group including a plurality of objects encoded in a Portable Document Format (PDF) document, by

blending a color of each pixel representing the respective object with a color of a pixel in the backdrop of the knockout group overlapped by the respective pixel independently of a shape of any of the objects of the knockout group, thereby producing an intermediate color of the respective pixel, and

computing a final color of each pixel representing the respective object as a function of the intermediate color, the color of the pixel in the backdrop prior to the blending, and a fractional coverage value of the shape of the respective object, thereby knocking out at least a portion of the pixel in the backdrop based on the shape of the respective object, the final color of each pixel being computed by adding a product of the fractional coverage value and the intermediate color to a product of a complement of the fractional coverage value and the color of the pixel in the backdrop;

storing the final color of each pixel in a framebuffer of the GPU for rendering on a display device; and

generating, by the GPU, a rendered image based on the final color of each pixel stored in the framebuffer.

8. The system of claim 7 , wherein the process further comprises generating data representing a single-sampled shape texture of the knockout group.

9. The system of claim 8 , wherein the process further comprises attaching the single-sampled shape texture of the knockout group to the framebuffer.

10. The system of claim 8 , wherein the process further comprises recording an accumulated shape of all of the objects of the knockout group in the single-sampled shape texture of the knockout group.

11. The system of claim 8 , wherein the process further comprises consolidating the single-sampled shape texture of the knockout group with a single-sampled shape texture of a parent knockout group including a different plurality of objects encoded in the PDF document.

12. The system of claim 11 , wherein the process further comprises compositing each object of the parent knockout group with a backdrop of the parent knockout group, wherein the backdrop of the parent knockout group comprises the composited objects and consolidated shapes of the knockout group.

13. A non-transitory computer readable medium having instructions encoded thereon that when executed by one or more computer processors cause the one or more computer processors to perform a process comprising:

compositing, by a graphics processing unit (GPU), each object of a knockout group with a backdrop of the knockout group, the knockout group including a plurality of objects encoded in a document, by

blending a color of each pixel representing the respective object with a color of a pixel in the backdrop of the knockout group overlapped by the respective pixel independently of a shape of any of the objects of the knockout group, thereby producing an intermediate color of the respective pixel, and

computing a final color of each pixel representing the respective object as a function of the intermediate color, the color of the pixel in the backdrop prior to the blending, and a fractional coverage value of the shape of the respective object, thereby knocking out at least a portion of the pixel in the backdrop based on the shape of the respective object, the final color of each pixel being computed by adding a product of the fractional coverage value and the intermediate color to a product of a complement of the fractional coverage value and the color of the pixel in the backdrop;

storing the final color of each pixel in a framebuffer of the GPU for rendering on a display device; and

generating, by the GPU, a rendered image based on the final color of each pixel stored in the framebuffer.

14. The non-transitory computer readable medium of claim 13 , wherein the process further comprises generating data representing a single-sampled shape texture of the knockout group.

15. The non-transitory computer readable medium of claim 14 , wherein the process further comprises attaching the single-sampled shape texture of the knockout group to the framebuffer.

16. The non-transitory computer readable medium of claim 14 , wherein the process further comprises recording an accumulated shape of all of the objects of the knockout group in the single-sampled shape texture of the knockout group.

17. The non-transitory computer readable medium of claim 14 , wherein the process further comprises consolidating the single-sampled shape texture of the knockout group with a single-sampled shape texture of a parent knockout group including a different plurality of objects encoded in the document.