Video preview frame thumbnail generation

Implementing Video Preview Frame Generation (Thumbnails)

Video previews needed in two scenarios: single frame as cover (poster for <video>) and frame grid for timeline (sprite sheet). Both built via FFmpeg, but logic slightly different.

Single Preview

Simplest variant — frame at specific timestamp:

ffmpeg -i input.mp4 -ss 00:00:05 -vframes 1 -q:v 2 thumbnail.jpg

-ss 00:00:05 — position (5 seconds from start). -vframes 1 — one frame. -q:v 2 — JPEG quality (1 = best, 31 = worst).

Better place mark not at 0 seconds (often black frame) and not at last second. Good heuristic — 10% of video length, but not less than 3 seconds and not more than 30.

Auto-select Informative Frame

FFmpeg can select frames by "most change" metric via thumbnail filter:

ffmpeg -i input.mp4 \
  -vf "thumbnail=300" \
  -vframes 1 \
  thumbnail.jpg

thumbnail=300 — analyzes every 300 frames and selects most representative. About 10–12 seconds at 24 fps. Slower than direct time selection, but result more informative.

PHP Service for Preview Generation

namespace App\Services;

class VideoThumbnailService
{
    public function generatePoster(
        string $videoPath,
        string $outputPath,
        ?float $offsetSeconds = null,
        int    $width         = 1280,
        int    $height        = 720
    ): void {
        $duration = $this->getVideoDuration($videoPath);
        $offset   = $offsetSeconds ?? max(3.0, $duration * 0.1);
        $offset   = min($offset, $duration - 1.0);

        $scaleFilter = "scale={$width}:{$height}:force_original_aspect_ratio=decrease,"
            . "pad={$width}:{$height}:(ow-iw)/2:(oh-ih)/2:black";

        $cmd = implode(' ', [
            'ffmpeg -y',
            '-ss ' . number_format($offset, 3, '.', ''),
            '-i ' . escapeshellarg($videoPath),
            '-vframes 1',
            "-vf " . escapeshellarg($scaleFilter),
            '-q:v 3',
            escapeshellarg($outputPath),
            '2>&1',
        ]);

        exec($cmd, $output, $exitCode);

        if ($exitCode !== 0 || !file_exists($outputPath)) {
            throw new \RuntimeException(
                "Thumbnail generation failed: " . implode("\n", $output)
            );
        }
    }

    public function generateSpriteSheet(
        string $videoPath,
        string $outputPath,
        int    $columns    = 10,
        int    $rows       = 10,
        int    $thumbWidth = 160
    ): array {
        $duration    = $this->getVideoDuration($videoPath);
        $totalFrames = $columns * $rows;
        $interval    = $duration / $totalFrames;

        // tile filter creates grid of frames
        $fps    = 1 / $interval;
        $filter = "fps={$fps},scale={$thumbWidth}:-1,tile={$columns}x{$rows}";

        $cmd = implode(' ', [
            'ffmpeg -y',
            '-i ' . escapeshellarg($videoPath),
            "-vf " . escapeshellarg($filter),
            '-vframes 1',
            '-q:v 5',
            escapeshellarg($outputPath),
            '2>&1',
        ]);

        exec($cmd, $output, $exitCode);

        if ($exitCode !== 0) {
            throw new \RuntimeException(implode("\n", $output));
        }

        return [
            'path'        => $outputPath,
            'columns'     => $columns,
            'rows'        => $rows,
            'thumb_width' => $thumbWidth,
            'interval'    => $interval,   // seconds between frames
            'duration'    => $duration,
        ];
    }

    public function getVideoDuration(string $path): float
    {
        $cmd    = "ffprobe -v error -show_entries format=duration -of csv=p=0 "
            . escapeshellarg($path);
        $output = shell_exec($cmd);
        return (float) trim($output ?? '0');
    }
}

Sprite Sheet for Timeline

Player shows user preview on progress bar hover — sprite sheet: one image with frame grid. Client shifts background-position depending on cursor position.

For 10-minute video, 10×10 grid, interval — 6 seconds. Thumb width 160px, grid 1600×900px (at 16:9 thumb).

JavaScript to calculate position:

function getThumbnailPosition(currentTime, spriteData) {
    const { columns, rows, thumbWidth, interval, duration } = spriteData;
    const thumbHeight = Math.round(thumbWidth * 9 / 16); // assume 16:9

    const frameIndex = Math.min(
        Math.floor(currentTime / interval),
        columns * rows - 1
    );

    const col = frameIndex % columns;
    const row = Math.floor(frameIndex / columns);

    return {
        x: col * thumbWidth,
        y: row * thumbHeight,
        width:  thumbWidth,
        height: thumbHeight,
    };
}

CSS:

.seek-preview {
    background-image: url('/storage/videos/sprite.jpg');
    background-repeat: no-repeat;
    width: 160px;
    height: 90px;
}

JS on progress bar hover:

const pos   = getThumbnailPosition(hoveredTime, spriteData);
preview.style.backgroundPosition = `-${pos.x}px -${pos.y}px`;

Generation in Job

// app/Jobs/GenerateVideoThumbnailsJob.php
class GenerateVideoThumbnailsJob implements ShouldQueue
{
    public int $timeout = 300;
    public int $tries   = 2;

    public function __construct(private int $videoId) {}

    public function handle(VideoThumbnailService $service): void
    {
        $video     = Video::findOrFail($this->videoId);
        $inputPath = Storage::disk('videos')->path($video->original_path);
        $dir       = Storage::disk('videos')->path('thumbnails/' . $video->id);

        @mkdir($dir, 0755, true);

        // Poster
        $posterPath = "{$dir}/poster.jpg";
        $service->generatePoster($inputPath, $posterPath);

        // Sprite sheet
        $spritePath = "{$dir}/sprite.jpg";
        $spriteData = $service->generateSpriteSheet($inputPath, $spritePath);

        $video->update([
            'poster_path'    => "thumbnails/{$video->id}/poster.jpg",
            'sprite_path'    => "thumbnails/{$video->id}/sprite.jpg",
            'sprite_data'    => $spriteData,
        ]);
    }
}

Database Storage

ALTER TABLE videos
    ADD COLUMN poster_path  VARCHAR(500),
    ADD COLUMN sprite_path  VARCHAR(500),
    ADD COLUMN sprite_data  JSONB;

sprite_data stores grid metadata — columns, rows, interval, thumb_width. Client gets them with video data and uses for progress bar preview rendering.

Timeline

Single poster generation, Job, model integration — 3–4 hours. Sprite sheet + client JS for progress bar — another 4–6 hours.