LMultiview Reference Manual & Grid Layout Guide

The LMultiview class is the high-performance multi-view video grid compositor of the now2sdk framework, derived from the LObject class. It allows developers to composit multiple video sources (such as LLive, LFile, LStreamPlay, etc.) into a single multi-source monitoring layout. It is commonly used in professional broadcast switchers to display all incoming cameras, preview feeds, and program feeds on a single monitor.

Under the hood, LMultiview utilizes an offscreen OpenGL context (QOpenGLContext and QOffscreenSurface) to render composite frames on the GPU. This architecture ensures high-performance blending, scaling, text rendering for channel labels, and live audio peak level (VU meter) processing without consuming CPU cycles.

1. Composition Architecture

graph TD
    A[LMultiview Composer Loop] --> B[Offscreen OpenGL FBO Renderer]
    B --> C[GLSL Fragment Shader Processing]
    C -->|1. Grid Cell Blending & Scaling| D[Composition Output]
    C -->|2. Tally Border Rendering| D
    C -->|3. Title Safe Channel Labels| D
    C -->|4. Dual-Channel VU Peak Meters| D
    
    H[MultiviewSink per Cell] -->|Peak Level Computation| I[Live VU Meters]
    I --> D

Key Composition Features:

• GLSL Shader Compositing: Multiple video textures are blended and composited on the GPU according to the computed grid coordinates.
• Tally Borders: Supports Program (Red border) and Preview (Green border) tally states for each cell in the layout.
• Text Labels: Renders text overlays for channel names dynamically using OpenGL text textures.
• Live VU Meters: Displays left/right channel peak audio levels on top of each cell, updating in real-time based on incoming audio samples.
• Dynamic Orientations: The layout can be arranged with Program/Preview on top, bottom, left, or right, with the grid cells filling the remaining space.

2. API Reference & Public Methods

📌 Layout & Setup

void setupGrid(MultiviewOrientation orientation, int baseCount);

Configures the overall layout style:

• orientation: Position of the main Program (PGM) and Preview (PVW) screens. Can be MultiviewOrientation::TOP, MultiviewOrientation::BOTTOM, MultiviewOrientation::LEFT, or MultiviewOrientation::RIGHT.
• baseCount: Number of rows (if TOP/BOTTOM) or columns (if LEFT/RIGHT) for the small grid area.

void setPVWSource(LObject* obj);

Sets the video source connected to the Preview (PVW) window.

void setPGMSource(LObject* obj);

Sets the video source connected to the Program (PGM) window.

📌 Grid Cells Management

void setGridPos(LObject* obj, int row, int col, int size = 0, const std::string& name = "");

Adds a video source to a specific position in the grid:

• obj: The video source (e.g. LLive).
• row, col: 1-based row and column coordinates.
• size: Cell dimension multiplier. 0 for 1x1 standard size, 1 for 2x2 larger size.
• name: Optional label name.

void removeGridPos(int row, int col);

Removes the source at the specified row and column.

void setGridPGM(int row, int col);

Sets the tally border of the cell at row, col to Program (Red).

void setGridPVW(int row, int col);

Sets the tally border of the cell at row, col to Preview (Green).

void clearTally(int row, int col);

Clears any active tally border on the cell at row, col.

📌 UI Overlay & Monitoring

void setGridName(int row, int col, const std::string& name);
void setGridName(const std::string& key, const std::string& name);

Sets the display label text for a grid cell.

std::vector<GridCellInfo> getLayoutInfo();

Returns a list of all active cells containing their positions, dimensions, tally status, and audio peak values.

void setVideoFormat(const videoFormatProps& props);
void getVideoFormat(videoFormatProps& props);

Configures the output resolution and frame rate of the multi-view output frame.

📌 Licensing

bool setLicense(const std::string& licenseKey);
bool isLicensed() const;

Configures and verifies the module license. If unlicensed, a watermark is automatically overlayed on the output frame.

3. Integration Example

This example demonstrates how to set up LMultiview with Preview, Program, and 8 camera sources arranged in a grid:

#include "LMultiview.h"
#include "LLive.h"
#include "LOutput.h"
#include <iostream>

void initializeMultiviewSystem() {
    LMultiview* multiview = new LMultiview();
    
    // Set output format to 1080p 50fps
    videoFormatProps vProps;
    vProps.setVideoFormat = vF::fcc_1080p50;
    multiview->setVideoFormat(vProps);

    // Setup: Program/Preview on top, grid with 4 columns on the bottom
    multiview->setupGrid(MultiviewOrientation::TOP, 4);

    // Create 2 Main Outputs
    LLive* pgmCam = new LLive();
    LLive* pvwCam = new LLive();
    multiview->setPGMSource(pgmCam);
    multiview->setPVWSource(pvwCam);

    // Register 8 Camera Inputs
    for (int i = 0; i < 8; ++i) {
        LLive* cam = new LLive();
        // Position camera in a 2x4 grid on the bottom
        int row = (i / 4) + 1;
        int col = (i % 4) + 1;
        multiview->setGridPos(cam, row, col, 0, "CAM " + std::to_string(i + 1));
    }

    // Connect Multiview to Output (e.g. DeckLink or NDI streaming)
    LOutput* monitorOut = new LOutput();
    monitorOut->connect(multiview);
    monitorOut->Start();
    
    std::cout << "Multiview engine running..." << std::endl;
}