Overview

Migrate SpectrumWidget rendering from CPU-based QPainter to GPU-accelerated rendering via Qt6's QRhi abstraction layer. This offloads FFT spectrum, waterfall scrolling, and overlay compositing to the GPU, significantly reducing CPU load.

Motivation

Current SpectrumWidget::paintEvent runs entirely on the CPU:

• Waterfall scroll: CPU copies/shifts ~1MB QImage every row, 25x/sec — biggest CPU hog
• FFT polyline: QPainter software-rasterizes 1000+ point polyline each frame
• Overlays: filter passband, spots, TNFs, band plan — all alpha-blended in software

Estimated CPU reduction: 30-50% of SpectrumWidget CPU time, more significant at higher resolutions and FPS.

Why QRhi (not raw OpenGL or Vulkan)

QRhi is Qt6's render hardware interface — one API, correct backend selected per platform automatically. Avoids:

• OpenGL deprecation risk on macOS
• OpenGL ES vs desktop GL differences on Pi
• Raw Vulkan complexity for a simple rendering workload

GPU Rendering Plan

Waterfall (biggest win)

• Allocate a GPU texture (width × history_rows)
• New waterfall row → upload single texture row (~4KB), advance row index
• Render as a textured quad with UV offset for scrolling — zero CPU copy
• Color mapping via fragment shader (intensity → color LUT texture)

FFT Spectrum

• Upload bin values as a vertex buffer (~2000 floats)
• Draw as GL_LINE_STRIP with smoothing
• Grid lines as a separate line batch

Overlays

• Filter passband, spots, TNFs, band plan → textured/colored quads with alpha
• Text labels → pre-rendered texture atlas or continue using QPainter overlay on top of the GL surface

Hybrid Approach

Qt allows QPainter on top of a QRhiWidget for text and complex UI elements. Render the heavy stuff (waterfall texture, FFT polyline) on GPU, overlay text labels and spots with QPainter. This avoids the complexity of a text rendering pipeline on GPU.

Pop-out Consideration

If we use QRhi, popping out the spectrum to a floating window (#246) is cleaner than raw OpenGL — QRhi manages the render target, and creating a new swapchain on a new window doesn't require re-uploading textures.

Estimated Effort

Medium-high. ~300-400 lines for the QRhi rendering code, plus testing on all 4 platforms. The QPainter fallback should be kept as a compile-time option for headless/CI builds.

Dependencies

• Qt 6.6+ for stable QRhiWidget API
• No external libraries — QRhi is bundled with Qt6

Priority

Post-1.0. Current QPainter rendering works correctly on all platforms. This is a performance optimization, not a functional requirement.

73, Jeremy KK7GWY & Claude (AI dev partner)