replace -reverse with negative -speed

README.md

@@ -149,7 +149,7 @@ Often, animations appear to accelerate when frame are removed. This is not alway
 
 ## Trim Start / End
 
-The `-trimStart <n>` / `-trimEnd <n>` options drop `n` frames from the start and/or end of the original sequence.
+The `-trimStart <n>` / `-trimEnd <n>` options drop `n` frames from the start and/or end of the original sequence. Zero indicates no trimming.
 
 For brevity, we will now assume the None transition and elide the successive sequence repetitions.
 
@@ -181,7 +181,7 @@ With `-trimStart 1` and `-trimEnd 1`:
 
 ## Trim Edges
 
-For convenience, we provide a similar option `-trimEdges <n>`. This drops `n` frames from both sides of the original sequence.
+For convenience, we provide a similar option `-trimEdges <n>`. This drops `n` frames from both sides of the original sequence. Zero indicates no trimming.
 
 ### Before
 
@@ -199,7 +199,7 @@ With `-trimEdges 1`:
 
 ## Window
 
-The `-window <n>` option truncates the original sequence to a fixed frame count. This is helpful for cutting down long animations.
+The `-window <n>` option truncates the original sequence to a fixed frame count. This is helpful for cutting down long animations. Zero indicates no truncation.
 
 ### Before
 
@@ -225,7 +225,7 @@ With `-window 3` and `-trimStart 1`:
 
 The `-shift <offset>` option performs a circular, leftward shift on the original sequence. This is useful for fine tuning how the GIF's very first cycle presents, before entering successive loops.
 
-A negative offset indicates rightward shift. Zero is the neutral shift.
+Zero is the neutral shift. A negative offset indicates rightward shift.
 
 ### Before
 
@@ -247,25 +247,11 @@ With `-shift -1`:
 3 1 2
 ```
 
-# REVERSE
-
-The `-reverse` option reorders the original sequence backwards.
-
-### Before
-
-```text
-1 2 3
-```
-
-### After
-
-```text:
-3 2 1
-```
-
 # SPEED
 
-The `-speed <factor>` option adjusts animation speed. Speed is expressed as a factor relative to the original GIF frame delay. We recommend using values in the range `0.2` (slow) to `2.0` (fast).
+The `-speed <factor>` option adjusts animation speed. Speed is expressed as a factor relative to the original GIF frame delay. We recommend using magnitudes between `0.2` (slow) and `2.0` (fast).
+
+`1` = `1.0` is the neutral factor.
 
 We can diagram this in terms of the frame delays, expressed in centiseconds. That is, `4cs` indicates 4 centisec = 4/100 sec between advancing to the next frame.
 
@@ -289,7 +275,7 @@ With `-speed 0.5`:
 8cs 8cs 8cs
 ```
 
-`1.0` is the neutral factor. Speed cannot be zero or negative.
+A negative speed plays the original sequence in reverse order.
 
 ### Quirks, Quickly
 

config.go

@@ -15,9 +15,6 @@ import (
 
 // Config models a set of animation editing manipulations.
 type Config struct {
-	// Reverse plays the incoming sequence backwards (Default false).
-	Reverse bool
-
 	// TrimEdges removes frames from the start and end of the incoming sequence (Default zero).
 	TrimEdges int
 
@@ -38,7 +35,9 @@ type Config struct {
 	// Stitch denotes a loop continuity transition (Default Mirror).
 	Stitch Stitch
 
-	// Speed applies a factor to each frame delay (Default 1.0)
+	// Speed applies a factor to each frame delay (Default 1.0).
+	//
+	// Negative speed reverses the incoming sequence.
 	Speed float64
 }
 
@@ -68,8 +67,8 @@ func (o Config) Validate() error {
 		return errors.New("window cannot be negative")
 	}
 
-	if o.Speed <= 0 {
-		return errors.New("speed must be positive")
+	if o.Speed == 0 {
+		return errors.New("speed cannot be zero")
 	}
 
 	return nil
@@ -104,9 +103,12 @@ func (o Config) Edit(destPth string, sourceGif *gif.GIF) error {
 		clonePaletteds[i] = clonePaletted
 	}
 
-	if o.Reverse {
+	speed := o.Speed
+
+	if o.Speed < 0 {
 		ReverseSlice(clonePaletteds)
 		ReverseSlice(sourceDelays)
+		speed *= -1.0
 	}
 
 	clonePaletteds = clonePaletteds[o.TrimStart:]
@@ -158,7 +160,7 @@ func (o Config) Edit(destPth string, sourceGif *gif.GIF) error {
 
 		butteryPaletteds[i] = paletted
 		sourceDelay := sourceDelays[r]
-		butteryDelays[i] = int(math.Max(2.0, float64(sourceDelay)/o.Speed))
+		butteryDelays[i] = int(math.Max(2.0, float64(sourceDelay)/speed))
 
 		if o.Stitch == Mirror && i >= clonePalettedsLen-1 {
 			r--

src/cmd/buttery/main.go

@@ -18,9 +18,8 @@ var flagTrimStart = flag.Int("trimStart", 0, "drop frames from start of the inpu
 var flagTrimEnd = flag.Int("trimEnd", 0, "drop frames from end of the input GIF")
 var flagWindow = flag.Int("window", 0, "set fixed sequence length")
 var flagStitch = flag.String("stitch", "Mirror", "stitching strategy (None/Mirror/FlipH/FlipV)")
-var flagReverse = flag.Bool("reverse", false, "reverse original sequence")
 var flagShift = flag.Int("shift", 0, "rotate sequence left")
-var flagSpeed = flag.Float64("speed", 1.0, "speed factor (highly sensitive)")
+var flagSpeed = flag.Float64("speed", 1.0, "animation speed factor")
 var flagVersion = flag.Bool("version", false, "show version information")
 var flagHelp = flag.Bool("help", false, "show usage information")
 
@@ -81,7 +80,6 @@ func main() {
 	}
 
 	config := buttery.NewConfig()
-	config.Reverse = *flagReverse
 	config.TrimStart = *flagTrimStart + trimEdges
 	config.TrimEnd = *flagTrimEnd + trimEdges
 	config.Window = *flagWindow