External capture-card camera feed is degraded during export: stutter, color shift, and compression

[nathanael]

Description

Cap’s export pipeline appears to significantly degrade external camera/capture-card video sources, even when the live preview looks correct and the same source records cleanly in other macOS apps.

The problem is most visible with an HDMI capture-card camera source, but the issue seems to happen during Cap’s export/render stage rather than at capture/preview time.

In my case, the exported video has:

• noticeable stutter / choppy frame cadence in the camera/person video
• mismatched timing between the camera feed and the rest of the recording
• heavy compression artifacts, especially in the camera/background
• significant color shift, especially a strong red/magenta cast
• little or no visible improvement when choosing the highest available export quality settings

This makes Cap difficult to use for higher-quality product demos, walkthroughs, or client-facing recordings where a proper external camera is used.

Why I believe this is an export/render issue

I tested the same camera/capture setup through multiple paths:

Test setup

• macOS on Apple Silicon
• Sony camera over HDMI
• UGREEN 15389 / CM629 capture card
• Capture-card modes tested include 1080p and 1440p
• Cap Studio Mode export
• OBS comparison recording
• Photo Booth comparison recording
• Internal Sony SD card recording as reference

Results

1. Sony internal SD card recording
   • Best image quality
   • Expected sharpness and color
2. UGREEN capture card directly into Photo Booth
   • Good image quality
   • Slightly more compressed than the Sony internal recording, but acceptable
3. UGREEN capture card directly into OBS
   • Good image quality when manually selecting the correct capture mode, including 1440p
   • Comparable to Photo Booth
4. UGREEN capture card inside Cap preview
   • Preview appears mostly acceptable
5. Cap export
   • Camera feed becomes visibly worse
   • More compression
   • More red/magenta color shift
   • Choppy/stuttered camera motion
   • Export quality setting does not seem to materially improve the camera feed

This suggests the source feed itself is not the main problem. The same capture-card feed can look acceptable in Photo Booth and OBS. The degradation appears to happen when Cap renders/exports the final video.

Expected behavior

If the camera feed looks correct in preview, the exported file should preserve that quality reasonably well.

At minimum, Cap should provide export options that allow users to preserve camera quality, including:

• explicit bitrate control
• constant frame rate export
• source frame-rate preservation or proper frame-rate conversion
• color space / color range handling, especially Rec.709 limited/full range
• higher-quality camera-layer compositing
• an option for visually lossless or near-lossless export
• an option to export or access original captured tracks without destructive re-encoding

Actual behavior

The exported video is much worse than both:

• the live preview inside Cap
• the same capture source recorded in OBS or Photo Booth

The degradation includes:

• stutter in the camera/person video
• apparent frame-rate mismatch between input and export
• heavy camera-layer compression
• color shift toward red/magenta
• quality settings not producing a meaningful improvement

Why this matters

External cameras and HDMI capture cards are common for professional demos, tutorials, sales videos, education content, and client updates.

If Cap can preview the feed correctly but then exports a lower-quality, stuttered, color-shifted version, users cannot trust the final output for professional work.

This is especially problematic because Cap is positioned around polished local editing and export. The editor/preview may look usable, but the final exported file is not.

Related issues

This seems related to:

• Feature Request: RAW/Advanced export #703 RAW/Advanced export
• Export audio not sync to video #382 Export audio not sync to video
• "Minimal" quality option is misleading (seems to mean minimal compression) #1500 “Minimal” quality option is misleading

This issue may be broader than a feature request for RAW/Advanced export because it appears to involve a bug in the export/render pipeline for camera/capture-card sources.

Requested improvements

Ideally, Cap should add or improve:

1. Advanced export controls
   • bitrate
   • codec
   • constant frame rate vs source/passthrough frame rate
   • resolution
   • color range/color space
   • visually lossless option
2. Better external camera/capture-card handling
   • preserve the selected camera source resolution and frame rate
   • avoid unnecessary downsampling/recompression of the camera layer
   • correctly handle color range and color space from capture-card inputs
3. Diagnostic export info
   • show actual export resolution, FPS, bitrate, codec, and estimated file size
   • optionally show source camera resolution/FPS detected by Cap
4. No-transcode or minimally-transcoded export option
   • either direct access to original captured tracks
   • or a high-quality intermediate export for users who want to finish encoding in another tool

Reproduction steps

1. Connect an HDMI camera through a UGREEN 15389 / CM629 capture card on macOS.
2. Open Cap Studio Mode.
3. Select the capture card as the camera source.
4. Record screen + camera.
5. Confirm the preview looks acceptable.
6. Export using the highest available quality setting.
7. Compare the exported file against:
   • the same capture card recorded in Photo Booth
   • the same capture card recorded in OBS
   • the camera’s internal SD card recording

Result

The Cap export shows significantly worse camera quality than the other recordings, including stutter, compression, and color shift.

Additional note

This does not seem to happen the same way with a normal webcam source such as the Insta360 Link 2. The issue appears much worse with the HDMI capture-card source.

[linear]

CAP-730

[tembo]

Thanks for the detailed report and methodical testing across different apps. Your analysis pointing to the export/render pipeline rather than capture is consistent with what the code shows.

Looking at the codebase, there are a few specific areas that likely contribute to what you're seeing:

Frame timing and fallback behavior: The camera decoder in crates/rendering/src/lib.rs uses a max_fallback_distance of 2 frames when requesting camera frames during export. If your capture card delivers frames at a different rate than the export framerate, this can cause frame repetition or skipping, which would manifest as the choppy/stuttered motion you describe.

Color range handling: The H.264 encoder in crates/enc-ffmpeg/src/video/h264.rs hardcodes color::Range::MPEG (limited range, 16-235) and BT.709 colorspace. If your capture card outputs full-range RGB or a different color matrix, there's no detection or conversion happening before this gets applied. This mismatch between assumed and actual color range could explain the red/magenta color shift.

Bitrate allocation: The bitrate formula in the same file (around line 849) scales sublinearly with framerate - a 60fps source only gets about 1.18x the bitrate of 30fps. Combined with the fact that export quality settings control bits-per-pixel rather than absolute bitrate, the camera layer (which typically has more motion and detail than screen content) may be under-allocated.

Related issues: This connects to several open discussions:

• #703 covers RAW/advanced export - as noted in the comments there, you can access the raw media files by using "Open Package Contents" on the .cap bundle in Finder, which might be useful for diagnosing whether the degradation happens at capture or export
• #382 covers audio sync issues that share some root causes around timeline segment timing
• #1500 addresses the confusing "Minimal" quality label

For anyone investigating this further, the key files are:

• crates/rendering/src/layers/camera.rs - camera frame GPU processing and YUV conversion
• crates/export/src/mp4.rs - export pipeline orchestration
• crates/enc-ffmpeg/src/video/h264.rs - encoder configuration including color space settings

One diagnostic step that might help narrow this down: check the raw camera recording inside the .cap bundle (right-click > Show Package Contents) to see if the color shift and stutter are present in the source or only appear after export.