image/jpeg: replace fdct.go and idct.go with new implementation in dct.go
The fdct.go and idct.go files were derived from the MPEG-SSG and
JPEG-IJG reference code and therefore carry licenses specific to those
groups. Various license checkers flag these files as potentially
problematic. The code is also not terribly well documented.
This CL fixes the license problem by adding a new, from-scratch
implementation using a different algorithm. As a bonus, the new code
is both faster and more accurate than the old encumbered code.
On speed, the new code is up to 20% faster; benchmarks below.
On accuracy, in the set of blocks used in the test, we can measure
the number of output values that are off-by-one from the exact rounded answer.
The old FDCT was off in 8.6% of values; the new one is off in 2.5%.
The old IDCT was off in 1.4% of values; the new one is off in 1.2%.
goos: darwin
goarch: arm64
pkg: image/jpeg
cpu: Apple M3 Pro
│ old │ new │
│ sec/op │ sec/op vs base │
FDCT-12 619.6n ± 3% 586.5n ± 1% -5.34% (p=0.000 n=10)
IDCT-12 752.4n ± 4% 628.0n ± 1% -16.54% (p=0.000 n=10)
goos: linux
goarch: amd64
cpu: Intel(R) Xeon(R) CPU @ 2.30GHz
│ old │ new │
│ sec/op │ sec/op vs base │
FDCT-16 1.817µ ± 0% 1.542µ ± 0% -15.11% (p=0.000 n=10)
IDCT-16 1.897µ ± 0% 1.514µ ± 0% -20.22% (p=0.000 n=10)
goos: linux
goarch: arm64
cpu: whatever gotip-linux-arm64 has
│ old │ new │
│ sec/op │ sec/op vs base │
FDCT-8 1.844µ ± 0% 1.847µ ± 0% +0.14% (p=0.000 n=10)
IDCT-8 2.127µ ± 0% 1.973µ ± 0% -7.26% (p=0.000 n=10)
Change-Id: Ie6d14103c8478ba5a779f234da84f345828eb925
Reviewed-on: https://go-review.googlesource.com/c/go/+/705518
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Russ Cox <rsc@golang.org> Reviewed-by: Nigel Tao <nigeltao@google.com>