Writing a Reproducible Image Export Regression Fixture in TypeScript

Overview

Creating a reproducible image export regression fixture in TypeScript allows developers to test and verify the consistency of image outputs across different environments and configurations. This fixture ensures that the exported image remains the same, even when the input data or rendering settings change. This tutorial provides a complete TypeScript implementation that can be used in a browser environment to test image exports.

Step-by-Step Implementation

1. Set Up the HTML Document
   Create an HTML file with a canvas element where the image will be rendered. This canvas serves as the source for the image export.

2. Initialize the Canvas and Context
   Use JavaScript to access the canvas element and obtain its 2D rendering context. This context is used to draw the image and perform any necessary transformations.

3. Define the Image Data
   Create an object that holds the image data, including the source image, dimensions, and any additional styling or filters. This data is used to generate the image for export.

4. Draw the Image on the Canvas
   Use the canvas context to draw the image onto the canvas. Ensure that the image is scaled and positioned correctly to match the expected output.

5. Export the Image as a Data URL
   Convert the canvas content into a data URL using the toDataURL method. This data URL represents the image in a format that can be compared for consistency.

6. Compare the Exported Image with a Baseline
   Store a baseline image data URL (e.g., from a previous test run) and compare it with the newly exported image. If the data URLs match, the export is consistent; otherwise, it indicates a regression.

7. Handle Errors Gracefully
   Implement error handling for potential issues such as missing images, invalid dimensions, or unsupported image formats. Log appropriate error messages and ensure the fixture continues to run even if one test fails.

8. Run the Test Fixture
   Use a browser environment to execute the test fixture. Ensure that all dependencies are loaded, and the canvas is properly initialized before running the export and comparison steps.

Example Implementation

const canvas = document.getElementById('image-canvas') as HTMLCanvasElement;
const ctx = canvas.getContext('2d');

const imageData = {
  src: 'https://example.com/image.png',
  width: 500,
  height: 500,
  style: {
    filter: 'grayscale(100%)',
    scale: 1.5
  }
};

const image = new Image();
image.src = imageData.src;
image.onload = () => {
  ctx.drawImage(image, 0, 0, imageData.width, imageData.height);
  const dataUrl = canvas.toDataURL('image/png');
  // Compare with baseline dataUrl
  console.log('Exported Data URL:', dataUrl);
};

// Error handling
image.onerror = () => {
  console.error('Failed to load image');
};

Testing and Validation

To test the fixture, run the HTML file in a browser and observe the console output. If the exported image matches the baseline, the fixture passes. If not, it indicates a potential regression. Ensure that the test environment remains consistent to maintain the reliability of the fixture.

Conclusion

A reproducible image export regression fixture in TypeScript is a powerful tool for ensuring the consistency of image outputs. By following the steps outlined in this tutorial, developers can create a reliable test fixture that helps identify and prevent regressions in their image export processes.

Optional background preparation

If the source image also needs a transparent background, RemovePhotoBG can be used before the workflow described above. Keep an untouched original, inspect the cutout edges on the intended background and check the exported file before uploading it.