How to Create a Draggable Element on Top of a PDF Using PDF.js and jQuery

Описание: Learn how to add a draggable image stamp on a PDF using PDF.js and jQuery without affecting the underlying PDF display.
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How to Create a Draggable Element on Top of a PDF Using PDF.js and jQuery

Working with PDFs in web applications often requires more than just displaying them. You may want to enable users to interact with PDFs by adding elements like stamps or notes. If you've been trying to create a draggable object on top of a PDF using PDF.js and jQuery but encountered problems – like leaving a trail of previous objects or clearing the entire canvas – you're not alone. This guide will guide you through an efficient solution to implement a draggable element without disrupting the PDF content beneath it.

The Problem

You have successfully loaded a PDF using PDF.js, and you're able to render objects on top of it. However, when dragging an object, previous positions remain visible, creating an unsightly trail. The issue arises when you clear the canvas using the context.clearRect() method, which also clears the underlying PDF.

Your goal is clear: you want to drag an object while preserving the PDF content shown on the canvas.

The Solution

To solve this problem, we will use two separate canvases: one for rendering the PDF and the other for drawing the draggable object. By layering the canvases, you can clear and redraw the draggable object without affecting the PDF content below it.

Step 1: HTML Structure

Create an HTML structure with two canvas elements, making sure the second canvas overlaps the first:

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Step 2: Positioning Canvases with CSS

To ensure the canvases are stacked correctly, use some CSS for positioning:

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Step 3: JavaScript Variables and Setup

Define the necessary global variables to maintain the state of the draggable object. This includes its position, dimensions, and drawing context.

[[See Video to Reveal this Text or Code Snippet]]

Step 4: Loading the PDF

Next, implement the function to load the PDF into the bottom canvas. Ensure that the dimensions of the draggable canvas match those of the PDF canvas.

[[See Video to Reveal this Text or Code Snippet]]

Step 5: Drawing the Draggable Element

Create a function for drawing the draggable element on the second canvas without affecting the PDF:

[[See Video to Reveal this Text or Code Snippet]]

Step 6: Handling Mouse Events

Capture mouse events to move the object. Ensure that movement of the mouse only updates the position of the element when the mouse is pressed down:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By using two canvases layered over each other, you've effectively achieved the ability to drag an element across a PDF without ruining the underlying content. This method not only enhances the visual experience but also allows clear separation of responsibilities for rendering the PDF and dynamic elements.

Now you're ready to implement this functionality in your own web applications! Happy coding!