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redact: decode a JPEG with the fleet's decoder, the one the reader is shown #89

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Merged
tannevaled merged 1 commit into from
Nov 11, 2026
Merged

redact: decode a JPEG with the fleet's decoder, the one the reader is shown #89

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2 changes: 1 addition & 1 deletion go.mod
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Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,7 @@ go 1.27.2
require (
github.com/go-gfx/gfx v0.41.0
github.com/go-images/images v0.3.0
github.com/go-images/jpeg v0.3.0
github.com/go-ooxml/ooxml v0.7.0
github.com/go-ooxml/opc v0.2.1
github.com/go-pdfkit/document v0.2.0
Expand All @@ -28,7 +29,6 @@ require (
github.com/go-browserhttp/browserhttp v0.2.0 // indirect
github.com/go-gfx/oksvg v0.2.0 // indirect
github.com/go-images/gif v0.2.0 // indirect
github.com/go-images/jpeg v0.3.0 // indirect
github.com/go-images/jpeg2000 v0.14.0 // indirect
github.com/go-images/png v0.2.0 // indirect
github.com/go-opentype/fonts v0.14.0 // indirect
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15 changes: 14 additions & 1 deletion redactimage.go
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Expand Up @@ -5,8 +5,21 @@ import (
"fmt"
"image"
"image/draw"
"image/jpeg"

// ⛔ THE FLEET'S DECODER, NOT THE STANDARD LIBRARY'S. What this promises to
// write back is "what a reader would have seen", and the reader is
// go-pdfkit/render, which decodes /DCTDecode with this fork: Go's own
// image/jpeg with a four-component picture's chroma upsampled the way
// libjpeg does rather than by repeating each sample. Decoding here with a
// different decoder would make that promise true of a picture nobody was
// shown.
//
// ⛔ AND IT IS NOT ABOUT THE BINARY. This was first written claiming it
// saved linking a second decoder; `go list -deps` says otherwise, before
// and after. image/jpeg stays, because go-gfx/gfx/codec, go-images/images
// and go-pdfkit/html2pdf each import it — go-images/images imports BOTH.
// That is a wider divergence than this file, filed rather than fixed here.
"github.com/go-images/jpeg"
"github.com/go-pdfkit/extract"
"github.com/go-pdfkit/reader"
)
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9 changes: 8 additions & 1 deletion redactjpeg_test.go
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Expand Up @@ -130,7 +130,14 @@ var bottomQuarter = Redaction{Page: 1, Quads: []Quad{{
//
// ⛔⛔ THE KEPT SAMPLES ARE COMPARED AGAINST THIS TEST'S OWN DECODE OF THE
// ORIGINAL JPEG, BYTE FOR BYTE. That is the whole claim the lossless path
// makes. Asserting a colour instead would be asserting what an encoder does
// makes.
//
// ⛔ AND THE TWO DECODERS ARE NOT THE SAME ONE. This file encodes and decodes
// with the standard library's image/jpeg; redactimage.go decodes with
// go-images/jpeg, the fork go-pdfkit/render shows the reader. So the equality
// below is also the statement that the two agree on a three-component picture
// — and the day they stop agreeing, this goes red and names the row and column
// where. Importing the fork here as well would have thrown that away. Asserting a colour instead would be asserting what an encoder does
// with it, which is a different subject and a moving one; and asserting "it
// still looks about the same" would pass for a picture re-encoded into mud —
// which is what re-encoding as a JPEG would have done to every block of it,
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