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Interactive Mode

Renderer mode 3 launches an SDL2 window where you can explore any scene in real time with mouse-driven camera controls and an ImGui control panel.

Starting interactive mode

./rayon --scene ../resources/scenes/default_scene.yaml --start-samples 32 --target-fps 60
# > Choose renderer: 3

Or with the default built-in scene:

./rayon --start-samples 8 --adaptive-depth
# > 3

SDL2 required

Mode 3 is only available when SDL2 is found at CMake configure time. If the menu does not show option 3, rebuild with SDL2 installed.

Camera controls

Input Effect
Left mouse + drag Orbit — rotate the camera around the look-at point
Right mouse + drag Pan — translate both camera and look-at laterally
Scroll wheel Zoom — change distance from look-at point
Space Force re-render (reset accumulation, restart at --start-samples)
ESC Quit

Any camera change immediately resets sample accumulation. The image restarts at --start-samples SPP and accumulates from there.

Command-line flags for interactive mode

Flag Default Effect
--start-samples <n> 32 SPP when starting or after any camera movement
--samples-per-batch <n> 50 Quality ceiling: the auto-scheduler will never exceed this many samples per batch
--target-fps <fps> 60 Minimum frame rate target — the batch size auto-scales every frame to stay at or above this
--adaptive-depth off Progressively increase max bounce depth per stage
--no-auto-accumulate off Disable automatic sample increase when stationary

Example: maximum quality accumulation:

./rayon --scene ../resources/scenes/09_color_bleed_box.yaml \
        --start-samples 8 --target-fps 30 --adaptive-depth

Example: very responsive orbit for scene exploration:

./rayon --start-samples 4 --target-fps 120 --no-auto-accumulate

ImGui control panel

While the scene is rendering, a panel in the top-right corner of the window provides live controls:

Control Effect Notes
Samples slider Quality ceiling for the adaptive batch scheduler (--samples-per-batch) Does not trigger a full reset; auto-scheduler immediately respects the new ceiling
Max samples slider Upper limit for auto-accumulate —
Light intensity Scales area light emission No reset required — updates via cudaMemcpyToSymbol
Roughness Material roughness Triggers reset
Aperture DOF disk radius Triggers reset
Focus distance Thin lens focal plane Triggers reset
Max depth Ray bounce limit Triggers reset

Parameters that update without a reset (light intensity, exposure) are injected into the running kernel via cudaMemcpyToSymbol — no GPU scene rebuild.

Display pipeline

flowchart TD
    A["GPU kernel\n(CUDA)"] -->|accumulates samples| B["Float accum buffer\n(device memory)"]
    B -->|gamma correction kernel| C["uint8 display buffer\n(device memory)"]
    C -->|cudaMemcpy D→H\n3 B/pixel| D["uint8 host buffer\n(CPU RAM)"]
    D -->|SDL_UpdateTexture| E["SDL texture\n(GPU VRAM)"]
    E -->|SDL_RenderCopy| F["SDL renderer"]
    F -->|SDL_GL_SwapWindow| G["Display"]

Only 3 bytes per pixel cross the PCIe bus each frame. At 1920×1080, that is ~6 MB per frame — easily handled at 60 fps. Parameters that update without a full reset (light intensity, exposure) are injected via cudaMemcpyToSymbol directly into the running kernel — no GPU scene rebuild needed.

Sample screenshot

Interactive SDL2 window showing path-traced scene with Dear ImGui overlay

Interactive mode at 2048 SPP after 20 seconds of stillness. The ImGui panel (right) shows current samples, frame time, and live parameter sliders.