How to Generate Video Thumbnails in Dart Using File Picker

Creating a video player that includes a thumbnail feature can significantly enhance user experience. If you're developing an application in Dart and are managing videos, it's essential to know how to generate a thumbnail from a selected video file. In this article, we’ll look into the process of picking a video file using Dart’s file picker, and how to generate a thumbnail from that video. Understanding Video Thumbnail Generation Generating a thumbnail from a video involves extracting a frame from the video at a specified time. This typically happens after the user selects a video file. The FileUploadInputElement that you’re using in your existing code allows you to select files, but generating a thumbnail will require some additional processing. Let's dive right into how this can be achieved. Step 1: Modify Your File Picker Code Before generating the thumbnail, ensure your file picker correctly identifies video files. You can modify your existing file picker code as follows: import 'dart:html' as html; import 'dart:typed_data'; import 'dart:async'; import 'package:http/http.dart' as http; Future startWebFilePicker() async { html.InputElement uploadInput = html.FileUploadInputElement(); uploadInput.accept = 'video/'; // Accept only video files uploadInput.sayMultiple = false; uploadInput.draggable = true; uploadInput.click(); uploadInput.onChange.listen((e) { final files = uploadInput.files; if (files.isEmpty) return; final file = files[0]; generateThumbnail(file); }); } In this modification, I've added accept = 'video/' to ensure that only video files are picked. Step 2: Generate the Thumbnail To generate the thumbnail for the video, we will use an HTML element. This element can load the video file and allow us to capture a frame at a given time. Here’s how to do it: void generateThumbnail(html.File videoFile) { // Create a URL for the video file final url = html.Url.createObjectUrl(videoFile); final video = html.VideoElement() ..src = url ..currentTime = 2; // Time in seconds where the thumbnail is fetched // Wait until the video is loaded to prevent errors video.onLoadedMetadata.listen((event) { video.pause(); captureFrame(video); }); // Append to body for loading html.document.body.append(video); } void captureFrame(html.VideoElement video) { final canvas = html.CanvasElement(); canvas.width = 200; // Set desired thumbnail width canvas.height = 100; // Set desired thumbnail height final context = canvas.context2D; context.drawImage(video, 0, 0, canvas.width, canvas.height); // Create an image from the canvas final thumbnailUrl = canvas.toDataUrl('image/png'); displayThumbnail(thumbnailUrl); } void displayThumbnail(String thumbnailUrl) { final img = html.ImageElement(src: thumbnailUrl); img.width = 200; // Set desired image width img.height = 100; // Set desired image height // Append the thumbnail image to the DOM html.document.body.append(img); } Explanation of the Thumbnail Generation Process Creating Video Element: We create a element and set its source to the selected video file URL. Setting Current Time: We manually specify the time from which we want to capture the frame by setting currentTime. Here, I'm using 2 seconds. Loading Metadata: Once the video's metadata is loaded, we can safely capture a frame without issues. Canvas for Capture: We use an HTML element to draw the frame captured from the video. Displaying the Thumbnail: Finally, we convert the canvas content to a Data URL and create an image element, which we then append to the document body. Frequently Asked Questions (FAQ) Q: Can I change where the thumbnail is captured from? A: Yes, by adjusting the video.currentTime value, you can change the point from which the thumbnail is captured. Q: What file formats can I use? A: You can use any video file format supported by the browser, but the most common ones are MP4, WebM, and Ogg. Conclusion Generating a video thumbnail in Dart involves selecting the video through a file picker, creating a video element, capturing a frame, and displaying it. The provided code snippets will get you started on adding this feature to your video player. By enhancing your video player with thumbnails, you improve the user experience, allowing for a more engaging application. Let’s get coding!

May 12, 2025 - 03:54

How to Generate Video Thumbnails in Dart Using File Picker

Creating a video player that includes a thumbnail feature can significantly enhance user experience. If you're developing an application in Dart and are managing videos, it's essential to know how to generate a thumbnail from a selected video file. In this article, we’ll look into the process of picking a video file using Dart’s file picker, and how to generate a thumbnail from that video.

Understanding Video Thumbnail Generation

Generating a thumbnail from a video involves extracting a frame from the video at a specified time. This typically happens after the user selects a video file. The FileUploadInputElement that you’re using in your existing code allows you to select files, but generating a thumbnail will require some additional processing. Let's dive right into how this can be achieved.

Step 1: Modify Your File Picker Code

Before generating the thumbnail, ensure your file picker correctly identifies video files. You can modify your existing file picker code as follows:

import 'dart:html' as html;
import 'dart:typed_data';
import 'dart:async';
import 'package:http/http.dart' as http;

Future startWebFilePicker() async {
  html.InputElement uploadInput = html.FileUploadInputElement();
  uploadInput.accept = 'video/*'; // Accept only video files
  uploadInput.sayMultiple = false;
  uploadInput.draggable = true;
  uploadInput.click();

  uploadInput.onChange.listen((e) {
    final files = uploadInput.files;
    if (files.isEmpty) return;
    final file = files[0];
    generateThumbnail(file);
  });
}

In this modification, I've added accept = 'video/*' to ensure that only video files are picked.

Step 2: Generate the Thumbnail

To generate the thumbnail for the video, we will use an HTML element. This element can load the video file and allow us to capture a frame at a given time. Here’s how to do it:

void generateThumbnail(html.File videoFile) {
  // Create a URL for the video file
  final url = html.Url.createObjectUrl(videoFile);
  final video = html.VideoElement()
    ..src = url
    ..currentTime = 2; // Time in seconds where the thumbnail is fetched

  // Wait until the video is loaded to prevent errors
  video.onLoadedMetadata.listen((event) {
    video.pause();
    captureFrame(video);
  });

  // Append to body for loading
  html.document.body.append(video);
}

void captureFrame(html.VideoElement video) {
  final canvas = html.CanvasElement();
  canvas.width = 200; // Set desired thumbnail width
  canvas.height = 100; // Set desired thumbnail height
  final context = canvas.context2D;
  context.drawImage(video, 0, 0, canvas.width, canvas.height);

  // Create an image from the canvas
  final thumbnailUrl = canvas.toDataUrl('image/png');
  displayThumbnail(thumbnailUrl);
}

void displayThumbnail(String thumbnailUrl) {
  final img = html.ImageElement(src: thumbnailUrl);
  img.width = 200; // Set desired image width
  img.height = 100; // Set desired image height

  // Append the thumbnail image to the DOM
  html.document.body.append(img);
}

Explanation of the Thumbnail Generation Process

Creating Video Element: We create a element and set its source to the selected video file URL.
Setting Current Time: We manually specify the time from which we want to capture the frame by setting currentTime. Here, I'm using 2 seconds.
Loading Metadata: Once the video's metadata is loaded, we can safely capture a frame without issues.
Canvas for Capture: We use an HTML element to draw the frame captured from the video.
Displaying the Thumbnail: Finally, we convert the canvas content to a Data URL and create an image element, which we then append to the document body.

Frequently Asked Questions (FAQ)

Q: Can I change where the thumbnail is captured from?
A: Yes, by adjusting the video.currentTime value, you can change the point from which the thumbnail is captured.

Q: What file formats can I use?
A: You can use any video file format supported by the browser, but the most common ones are MP4, WebM, and Ogg.

Conclusion

Generating a video thumbnail in Dart involves selecting the video through a file picker, creating a video element, capturing a frame, and displaying it. The provided code snippets will get you started on adding this feature to your video player. By enhancing your video player with thumbnails, you improve the user experience, allowing for a more engaging application. Let’s get coding!