1. A system comprising: a first processing unit; a first shared memory accessible by the first processing unit; and a second shared memory accessible by the first processing unit, wherein the first processing unit is to: receive a first image data value of a digital image; determine a first index based on the first image data value; determine a value stored in a first array of the first shared memory at the first index; determine if the value stored in the first array of the first shared memory at the first index is equal to the first image data value; if the value stored in the first array of the first shared memory at the first index is equal to the first image data value, add 1 to a count value stored in a second array of the first shared memory at the first index; and if the value stored in the first array of the first shared memory at the first index is not equal to the first image data value, update a count value stored in the second shared memory in association with the first image data value.