An AVIF or a JPEG XL cut short loses the share of its picture asked for

CHANGELOG.md

@@ -43,6 +43,13 @@ because it turns other people's test suites red.
   padding, which busybox reads, and the padding file inside the archive takes
   the rest, as it did for larger archives. Those archives are not byte for
   byte what 0.5.0 wrote. Every other tar.gz is unchanged.
+- **A JPEG XL now opens in ffmpeg.** ffmpeg 7.1 never finished one, and
+  ffmpeg 9 took one below about 3.8 KB for a video and refused it, because of
+  the padding box behind the picture. The padding now stands in front of the
+  picture, in a box of the kind ffmpeg passes over, and nothing follows the
+  picture. Pillow, ImageMagick, Windows, Chrome and libjxl show the same
+  picture as before. Sizes are unchanged, but no JPEG XL is byte for byte what
+  0.5.0 wrote.
 
 ### Added
 
@@ -116,6 +123,14 @@ because it turns other people's test suites red.
   manifest says so: a few percent of a small PDF, falling in the index, and
   one keeping 10 billion bytes or more, which the index cannot reach. Files
   nothing cuts are unchanged.
+- **`truncate` cuts the picture of an AVIF or a JPEG XL by the percentage you
+  set.** A cut used to take only padding, leaving the whole picture. Now
+  `--damage truncate:missing=50` removes half of the coded picture, as an
+  interrupted download does. Chrome, Firefox, Windows, libavif and Pillow
+  refuse the cut AVIF. Half of a JPEG XL still shows the whole picture at
+  lower detail in Chrome and in libjxl with partial files allowed, because the
+  format sends a coarse picture first. A cut of 90% shows nothing. An AVIF
+  nothing cuts is unchanged. Every JPEG XL changes (see Breaking).
 
 - **tfg is on npm, for tests in Node.**
   `npm install --save-dev testing-files-generator` gives Playwright, Vitest

internal/format/avif/avif.go

@@ -35,7 +35,8 @@
 // sixty four bit box length that files above four gigabytes need. There is no
 // dead zone above the minimum. Pillow reads every one of them and refuses a
 // truncated or corrupted file, which is what makes it a witness. ffprobe reads
-// the broken ones too, so it is not.
+// the broken ones too, so it is not. A file a damage will cut has part of its
+// free boxes in front of the media data instead (cut.go).
 package avif
 
 import (
@@ -166,6 +167,9 @@ type memo struct {
 	// which is what makes every size above the minimum reachable a byte at a
 	// time.
 	total int64
+	// cutAt is where a damage will cut the file, zero when nothing does
+	// (format.Request.CutAt).
+	cutAt int64
 }
 
 func (generator) Plan(r format.Request) (format.Plan, error) {
@@ -187,7 +191,7 @@ func (generator) Plan(r format.Request) (format.Plan, error) {
 	if err := checkFits(r.Bytes, m); err != nil {
 		return format.Plan{}, err
 	}
-	m.total = r.Bytes
+	m.total, m.cutAt = r.Bytes, r.CutAt
 
 	p := format.Plan{
 		Bytes:       r.Bytes,
@@ -244,7 +248,7 @@ const holdsPerPixel = 8
 func checkFits(want int64, m memo) error {
 	floor := int64(minimumBytes)
 	reason := core.Says("avif.MinimumReason",
-		"the smallest picture this format draws codes to %d B at worst, and the file always carries a free box, which costs %d B even when it holds nothing",
+		"the smallest picture this format draws codes to %d B at worst, and the file always carries a padding box, which costs %d B even when it holds nothing",
 		core.A("Coded", minimumBytes-boxHeader), core.A("Box", boxHeader))
 
 	// A picture named by hand can be larger than the smallest one, and then its
@@ -254,7 +258,7 @@ func checkFits(want int64, m memo) error {
 		if own := int64(len(m.coded)) + boxHeader; own > floor {
 			floor = own
 			reason = core.Says("avif.MinimumReasonOwn",
-				"a %dx%d picture at quality %d codes to %d B, and the file always carries a free box, which costs %d B even when it holds nothing",
+				"a %dx%d picture at quality %d codes to %d B, and the file always carries a padding box, which costs %d B even when it holds nothing",
 				core.A("Width", m.width), core.A("Height", m.height), core.A("Quality", m.quality), core.A("Coded", len(m.coded)), core.A("Box", boxHeader))
 		}
 	}
@@ -491,6 +495,12 @@ func (generator) Write(ctx context.Context, w io.Writer, p format.Plan) error {
 			m.width, m.height, len(coded), m.total))
 	}
 
+	if m.cutAt > 0 {
+		if done, err := writeForACut(ctx, w, m, coded); done || err != nil {
+			return err
+		}
+	}
+
 	// The map: the coder's boxes, its media data the content, and the free
 	// boxes after them that make up the size.
 	markBoxes(w, coded)