image/draw: optimize DrawMask when drawing a Uniform Image onto a Paletted Image #35938

What version of Go are you using (go version)? 1.13.4

$ go version
go version devel +a18608a Mon Dec 2 20:12:54 2019 +0000 linux/amd64

Does this issue reproduce with the latest release?

Yes - and github master branch too

What operating system and processor architecture are you using (go env)?

$ go env
GOHOSTARCH="amd64"
GOHOSTOS="linux"

What did you do?

Wrote a program that ran much slower than expected. The following program will reproduce the slow speed:

package main

import (
	"fmt"
	"image"
	"image/color"
	"image/color/palette"
	"image/draw"
	"time"
)

func main() {
	startTime := time.Now()
	red := &color.RGBA{255, 0, 0, 0}
	rect := image.Rect(-1000, -1000, 1000, 1000)
	palettedImage := image.NewPaletted(rect, palette.WebSafe)
	uniformImage := image.NewUniform(red)
	draw.Draw(palettedImage, rect, uniformImage, image.Point{}, draw.Src)
	numPoints := rect.Dx() * rect.Dy()
	took := time.Since(startTime)
	fmt.Printf("Took: %s for %d points - %f pps\n", took, numPoints, float64(numPoints)/took.Seconds())
}

What did you expect to see?

It should run very quickly, not take several seconds. The DrawMask function in image/draw/draw.go is called a lot for the specific application I'm working with and this is the bottleneck.

What did you see instead?

I expected it to run nearly instantly

Here is a proposed fix:

diff --git a/src/image/draw/draw.go b/src/image/draw/draw.go
index 932a544..3a0157d 100644
--- a/src/image/draw/draw.go
+++ b/src/image/draw/draw.go
@@ -181,8 +181,25 @@ func DrawMask(dst Image, r image.Rectangle, src image.Image, sp image.Point, mas
                return
        case *image.Paletted:
                if op == Src && mask == nil && !processBackward(dst, r, src, sp) {
-                       drawPaletted(dst0, r, src, sp, false)
-                       return
+                        switch src0 := src.(type) {
+                        case *image.Uniform:
+                                i0 := dst0.PixOffset(r.Min.X, r.Min.Y)
+                                i1 := i0 + r.Dx()
+                                colorIdx := uint8(dst0.Palette.Index(src0.C))
+                                for i := i0; i < i1; i++ {
+                                        dst0.Pix[i] = colorIdx
+                                }
+                                firstRow := dst0.Pix[i0:i1]
+                                for y := r.Min.Y + 1; y < r.Max.Y; y++ {
+                                        i0 += dst0.Stride
+                                        i1 += dst0.Stride
+                                        copy(dst0.Pix[i0:i1], firstRow)
+                                }
+                                return
+                        default:
+                                drawPaletted(dst0, r, src, sp, false)
+                                return
+                        }
                }
        }