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Preserve canvas pixels in rendered clones #55

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8a3634f
Preserve canvas pixels in rendered clones
Jun 20, 2026
943e119
Make AppDir executable assertion POSIX-only
Jun 21, 2026
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35 changes: 35 additions & 0 deletions browser/canvas.go
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Original file line number Diff line number Diff line change
@@ -0,0 +1,35 @@
package browser

import "github.com/go-rod/rod"

func snapshotHTML(page *rod.Page) (string, error) {
if err := snapshotCanvasPixels(page); err != nil {
return "", err
}
return page.HTML()
}

func snapshotCanvasPixels(page *rod.Page) error {
_, err := page.Eval(`() => {
const maxCanvasSnapshotPixels = 4096 * 4096;
for (const canvas of document.querySelectorAll("canvas")) {
try {
if (canvas.width === 0 || canvas.height === 0) continue;
if (canvas.width * canvas.height > maxCanvasSnapshotPixels) continue;
const src = canvas.toDataURL("image/png");
if (!src.startsWith("data:image/png")) continue;
const img = document.createElement("img");
for (const attr of canvas.attributes) img.setAttribute(attr.name, attr.value);
img.setAttribute("src", src);
if (!img.hasAttribute("alt")) img.setAttribute("alt", "");
if (!img.hasAttribute("width")) img.setAttribute("width", String(canvas.width));
if (!img.hasAttribute("height")) img.setAttribute("height", String(canvas.height));
const rect = canvas.getBoundingClientRect();
if (!img.style.width && rect.width > 0) img.style.width = rect.width + "px";
if (!img.style.height && rect.height > 0) img.style.height = rect.height + "px";
canvas.replaceWith(img);
} catch (_) {}
}
}`)
return err
}
2 changes: 1 addition & 1 deletion browser/pool.go
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Original file line number Diff line number Diff line change
Expand Up @@ -139,7 +139,7 @@ func (p *Pool) Render(ctx context.Context, rawURL string) (RenderResult, error)
settle(page, p.opts.Settle)
}

html, err := page.HTML()
html, err := snapshotHTML(page)
if err != nil {
return RenderResult{}, fmt.Errorf("serialise %s: %w", rawURL, err)
}
Expand Down
54 changes: 54 additions & 0 deletions browser/pool_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -2,7 +2,9 @@ package browser

import (
"context"
"encoding/base64"
"errors"
"image/png"
"net/http"
"net/http/httptest"
"os"
Expand Down Expand Up @@ -126,6 +128,58 @@ func TestRenderCapturesFinalDOM(t *testing.T) {
}
}

func TestRenderPreservesCanvasPixels(t *testing.T) {
if testing.Short() {
t.Skip("render test drives Chrome; skipped under -short")
}
if _, ok := LookChrome(); !ok {
t.Skip("no Chrome/Chromium found; skipping render test")
}

srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
w.Header().Set("Content-Type", "text/html; charset=utf-8")
_, _ = w.Write([]byte(`<!doctype html><html><body>
<canvas id="graph" width="32" height="16"></canvas>
<script>
const ctx = document.getElementById("graph").getContext("2d");
ctx.fillStyle = "#ff0000";
ctx.fillRect(0, 0, 32, 16);
</script>
</body></html>`))
}))
defer srv.Close()

p := New(Options{Headless: true, Workers: 1, Settle: 300 * time.Millisecond, RenderTimeout: 20 * time.Second})
defer func() { _ = p.Close() }()

ctx, cancel := context.WithTimeout(context.Background(), 60*time.Second)
defer cancel()

res, err := p.Render(ctx, srv.URL)
if err != nil {
t.Fatalf("render: %v", err)
}
if !strings.Contains(res.HTML, `src="data:image/png;base64,`) {
t.Fatalf("rendered HTML does not include a canvas PNG snapshot:\n%s", res.HTML)
}
data, ok := strings.CutPrefix(res.HTML[strings.Index(res.HTML, `src="data:image/png;base64,`):], `src="data:image/png;base64,`)
if !ok {
t.Fatalf("rendered HTML does not include a canvas PNG snapshot:\n%s", res.HTML)
}
encoded, _, ok := strings.Cut(data, `"`)
if !ok {
t.Fatalf("canvas PNG snapshot is not quoted correctly:\n%s", res.HTML)
}
img, err := png.Decode(base64.NewDecoder(base64.StdEncoding, strings.NewReader(encoded)))
if err != nil {
t.Fatalf("decode canvas PNG snapshot: %v", err)
}
r, g, b, a := img.At(0, 0).RGBA()
if r < 0xff00 || g > 0x0100 || b > 0x0100 || a < 0xff00 {
t.Fatalf("canvas PNG first pixel = rgba(%#x, %#x, %#x, %#x); want opaque red", r, g, b, a)
}
}

func TestIsHTML(t *testing.T) {
cases := []struct {
ct string
Expand Down
3 changes: 2 additions & 1 deletion pack/appdir_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,7 @@ import (
"image/png"
"os"
"path/filepath"
"runtime"
"strings"
"testing"
)
Expand Down Expand Up @@ -60,7 +61,7 @@ func TestBuildAppDir(t *testing.T) {
if string(tr[:8]) != trailerMagic {
t.Error("AppRun missing the trailer")
}
if info, _ := os.Stat(apprun); info.Mode().Perm()&0o111 == 0 {
if info, _ := os.Stat(apprun); runtime.GOOS != "windows" && info.Mode().Perm()&0o111 == 0 {
t.Error("AppRun is not executable")
}

Expand Down