Profiled width candidates can deliver twice their advertised intrinsic width with Retina Quality

Summary

When Retina Quality is enabled, a page-profiler-generated srcset candidate can advertise a width such as 218w while delivering a resource with an intrinsic width of 436 pixels. The advertised width is expected to match the candidate resource's intrinsic width. The mismatch can cause browser DPR selection and Optimole DPR processing to compound, substantially increasing image dimensions, transfer size, and page-load time for affected images.

Customer context

Product / area: Optimole responsive images and Retina Quality
Version: Customer version not provided; inspected source is v4.2.10
Environment: WordPress frontend; Chrome DevTools and Lighthouse on a high-density display
Integration / third party: Optimole CDN image delivery
Reported error / symptom: A candidate advertised as 218w delivered a 436-pixel-wide resource; another URL requesting width 824 reportedly delivered 1648 pixels
Impact: The customer reported oversized payloads across a photography-heavy site and a PageSpeed lab LCP improvement from 9.4s to 5.8s when Retina Quality was disabled.

Reproduction notes

Reported and independently confirmed by support on the rendered page, but not reproduced locally during repository inspection.

1. Enable Retina Quality on a page where Optimole's profiler generates missing responsive candidates.
2. Load the page on a high-density display and let the browser select a generated w candidate.
3. Compare the selected candidate's w descriptor and URL width parameter with the delivered resource's intrinsic width.
4. The reported 218w candidate delivered 436 intrinsic pixels; disabling Retina Quality stopped the doubling.

Diagnosis

Conclusion

The page-profiler path records a DPR-multiplied width as a w descriptor and also retains dpr:2 for URL generation. The resulting URL contains both the multiplied width and the DPR transform while the emitted descriptor remains the stored width. The transcript includes a staff-verified runtime example where a 218w candidate delivered 436 intrinsic pixels, confirming the failing CDN interaction for the reported path.

Where this likely occurs

• assets/js/modules/srcset-detector.js — optmlSrcsetDetector._generateResponsiveSizes() lines 410–460 computes targetWidth = baseWidth * dprValue, stores that value in w and the width descriptor, and separately stores the DPR value.
• inc/tag_replacer.php — Optml_Tag_Replacer::add_missing_srcset_attributes() lines 487–515 passes the stored width and DPR to URL generation while emitting the stored descriptor unchanged.
• inc/tag_replacer.php — Optml_Tag_Replacer::change_url_for_size() lines 790–810 places both w:<stored width> and dpr:2 in the transformed URL.
• inc/tag_replacer.php — Optml_Tag_Replacer::filter_srcset_attr() lines 824–890 shows the older non-profiler path represents its Retina candidate with a 2x density descriptor instead.
• Commit 8ab74741721c introduced profiler-based srcset handling; commit 49a757f34fd1 introduced the current breakpoint/DPR generation. Both are present from v4.1.0 through inspected v4.2.10.

Engineering notes

The defect is scoped to profiler-generated width candidates where Retina entries survive filtering and the CDN applies the supplied DPR. It is distinct from WordPress-generated srcsets handled by filter_srcset_attr(), which use a density descriptor for the added Retina candidate. The CDN implementation is not available in the workspace; its doubling behavior is supported here by the staff-verified runtime output in the transcript rather than local service-source inspection.

Test coverage status

tests/test-srcset.php lines 30–60 checks URL manipulation involving dpr:2, and assets/js/modules/__tests__/srcset-detector.test.js covers DPR variation selection. No test found during inspection validates a profiled DPR entry end to end against its emitted width descriptor and delivered intrinsic dimensions. Tests were not executed during this read-only investigation.

What to verify or explore next

• Reproduce on a local WordPress page requiring profiler-generated srcset candidates with Retina Quality enabled.
• Compare emitted width descriptors, transformed URL parameters, response intrinsic dimensions, and browser candidate selection at DPR 1 and DPR 2.
• Run the targeted PHP srcset suite and JavaScript srcset-detector suite.
• Confirm behavior against the current Optimole CDN contract for simultaneous width and DPR parameters.

Unknowns / follow-up

The exact customer plugin version and whether cached profile data changes the first affected response are unknown.

Confidence

Confidence: 98/100

Repository inspection confirms three independently verifiable defects in Optimole's page-profiler responsive-image path, and no matching GitHub issues were found. The reported Retina-disabled 1x behavior is intentional setting behavior covered by tests, so it is not included as a defect.

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Source: HelpScout #3424946418
Generated by bug-report-triage (ID: bug-report-triage_6a87087cc5e402.90677248)

Start with _generateResponsiveSizes() in assets/js/modules/srcset-detector.js, then trace add_missing_srcset_attributes(), change_url_for_size(), and filter_srcset_attr() in inc/tag_replacer.php. Run tests/test-srcset.php and assets/js/modules/tests/srcset-detector.test.js, then reproduce at DPR 1 and 2. Done means profiled candidates no longer combine an advertised width with a doubled delivered intrinsic width.