In my Angular 6 application, I’m adding a file upload feature where the uploaded image should be automatically cropped and resized in the preview. I’m using a <canvas> element and trying to draw the image based on user selection.
Here’s what I have:
<canvas id="canvas"></canvas>
<div style="display:none;">
<img id="source" [src]="url" width="300" height="227">
</div>
<input type='file' (change)="onSelectFile($event)">
And the onSelectFile function:
onSelectFile(event) {
if (event.target.files && event.target.files[0]) {
const reader = new FileReader();
reader.readAsDataURL(event.target.files[0]);
reader.onload = (event) => {
this.url = event.target.result;
};
const canvas: any = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
const image = document.getElementById('source');
ctx.drawImage(image, 33, 71, 104, 124, 21, 20, 87, 104);
}
}
Even though I’m referencing examples like this JSFiddle( Edit fiddle - JSFiddle - Code Playground ) , the image doesn’t appear correctly when selected from the file input.
My goal is to allow users to choose a file and immediately preview a cropped and resized version using just plain JavaScript, no jQuery or external libraries.
How can I achieve this with a reliable JavaScript crop image approach that works with the file input and renders the cropped version correctly on the canvas?
The issue you’re facing is that the image is being drawn on the canvas before it’s fully loaded, which can cause the canvas to either be empty or display incorrect results. To fix this, you should ensure that the image is fully loaded before trying to draw it on the canvas.
Here’s how you can fix it: When the file is selected and loaded into the reader, create a new image object and set its src to the file’s data. But, instead of calling drawImage() immediately, wait until the image is completely loaded. You can achieve this by using the onload event of the image, ensuring that the canvas drawing only happens once the image is fully available.
In simple terms: First, load the image properly, and then draw it on the canvas once it’s ready. This way, you can control when the image appears and avoid errors caused by trying to use an image that hasn’t finished loading yet.
To implement an image crop and resize feature in Angular 6, it’s best to avoid directly manipulating DOM elements, as Angular’s rendering lifecycle might introduce timing issues. Instead, you can use an Image() instance to load the image and manage the cropping and resizing on a canvas.
Here’s the approach:
- Load the Image: When the user selects an image file, use a
FileReader to load the image as a Data URL. This allows you to preview the image without relying on DOM elements.
- Create an Image Instance: Once the image is loaded, create a new
Image() instance. This gives you better control over the image loading process.
- Render on Canvas: Use a
<canvas> element to display and manipulate the image. You can specify the crop area and the final size on the canvas, ensuring the image is resized and cropped accordingly.
By following this method, you ensure that the image is processed smoothly and without timing issues, as Angular handles the data separately from the image manipulation process. This results in a more reliable image cropping and resizing feature in your Angular application.
The issue you’re facing with image cropping in Angular 6 is likely due to the timing of when the drawImage() function is called. This function may be executed before the image is fully loaded, which causes errors. A cleaner approach would be to avoid referencing the DOM element (#source) directly and instead handle everything with an Image() object.
By creating a new Image() object and ensuring it is fully loaded before drawing, you can control the image processing more reliably. Here’s the process:
- Load the Image: Use a
FileReader to load the selected image as a Data URL.
- Create an Image Object: Instead of referring to a DOM element, create a new
Image() instance. This allows you to have better control over when the image is fully loaded.
- Draw the Image: Once the image is loaded, use a
<canvas> to crop and resize it. You can specify the exact area of the image you want to crop and the final size to be displayed.
This method ensures the image is processed correctly without Angular’s rendering lifecycle interfering.