Fix GStreamer pipeline for MJPG v4l2 capture

[PeteHaughie]

Summary

ofGstVideoGrabber::setup() builds a broken GStreamer pipeline for MJPG (v4l2) cameras, so capture fails on Linux with Internal data stream error. Two independent defects in the pipeline string builder:

1. decodebin appended after jpegdec. The decoder selection is:

   if (format.mimetype == "image/jpeg")    decodebin  = "! jpegdec ";
   if (format.mimetype == "video/x-bayer") decodebin  = "! bayer2rgb ";
   else if (encoded/unknown)               decodebin += "! decodebin ";

   The second if is not chained to the jpeg branch, and an MJPG device's format_name ("MJPG") maps to GST_VIDEO_FORMAT_UNKNOWN, so "! decodebin " is appended after jpegdec. decodebin cannot link the raw video jpegdec emits -> streaming stopped, reason not-linked (-1).

2. videoscale overwrites videoconvert. When the requested size differs from the selected device format:

   if (format_name != getGstFormatName(internalPixelFormat)) scale  = "! videoconvert ";
   if (w != format.width || h != format.height)              scale  = "! videoscale method=2 ";  // clobbers the above

   With internalPixelFormat defaulting to OF_PIXELS_RGB, the appsink's RGB caps are then never satisfied -> reason not-negotiated (-4).

Reproduction

Any Linux v4l2 camera that exposes MJPG, when the size passed to ofVideoGrabber::setup(w, h) differs from the format OF selects. Here: a USB capture card offering 720x576 MJPG, asked for 640x480.

Pipelines (from the ofGstUtils log):

• before: v4l2src ! image/jpeg,width=720,height=576,framerate=30/1 ! jpegdec ! decodebin ! videoscale method=2 ! appsink caps="video/x-raw, format=RGB, width=640, height=480" -> not-linked
• fix 1 only: jpegdec ! videoscale ! appsink(RGB) -> not-negotiated
• after: jpegdec ! videoconvert ! videoscale method=2 ! appsink(RGB) -> streams

Verified with gst-launch-1.0 against the same device, and end-to-end on a Raspberry Pi 3 (headless KMS/GBM) with a MacroSilicon 534d:2109 UVC capture device.

Notes

• Platform-independent; affects any GStreamer 1.x Linux build.
• Only reachable when internalPixelFormat != OF_PIXELS_NATIVE (the default OF_PIXELS_RGB path).

[Copilot]

Copilot review overview

🟢 Approval recommended

Both targeted changes correctly address the described pipeline failures without introducing unresolved issues.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes GStreamer pipeline construction for MJPG V4L2 capture.

Changes:

• Prevents decodebin from being appended after jpegdec.
• Preserves videoconvert when adding videoscale.

File	Description
libs/​openFrameworks/​video/​ofGstVideoGrabber.cpp	Corrects decoder selection and composes conversion/scaling stages.

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[danoli3]

thanks!