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Fabric.js: Why Group Offset Doesn't Work When Loading from JSON

Learn why group object offset (left and top) doesn't apply when loading from JSON in fabric.js. Complete guide to solving group positioning issues.

#deserialization#fabric-js#group-offset#json-loading#object-groups#positioning

Why doesn’t the offset of group objects (left and top) work in fabric.js when loading from JSON?

I’m creating a group of objects through a PHP array and passing it to fabric.js via JSON. Inside the group, I set the offset using the left and top properties, but the changes aren’t applied. I can’t find the reason why the offset isn’t working.

Here’s my PHP code for creating the group:

php
$horizontalCreasing = [
    'type' => 'group',
    "name" => "horizontal_creasing",
    // offset down
    'left' => 0,
    'top' => self::$height,
    'objects' => [],
];

// Object inside
$crease_5 = [
    'type' => 'line',
    'x1' => 0,
    'y1' => 0,
    'x2' => self::$length - self::$thickness,
    'y2' => 0,
    'stroke' => self::$creasingColor,
    'strokeWidth' => 2,
];
array_push($horizontalCreasing['objects'], $crease_5);

Here’s the HTML canvas element:

html
<canvas id="canvas" width="1000" height="600" data-dieline="{{ json_encode($dieline, JSON_HEX_QUOT|JSON_HEX_APOS) }}"></canvas>

Here’s the JavaScript code for loading data into fabric.js:

javascript
import * as fabric from 'fabric';

// Load data
let dataCanvas = document.getElementById('canvas');
let dieline = dataCanvas.dataset.dieline;
let json = JSON.parse(dieline);

// Fabric.js
const canvas = new fabric.StaticCanvas('canvas');

// Required redraw after loading
canvas.loadFromJSON(json, function() {
    canvas.requestRenderAll();
    canvas.toSVG();
});

How do I properly set the offset for group objects when loading from JSON in fabric.js?

NeuroAgent

The main issue is that when loading a group from JSON into fabric.js, the group’s offset (left and top) may not be applied correctly due to specifics of the deserialization mechanism and timing of property application.

Table of Contents

Main causes of the problem

Timing synchronization issue

As research from StackOverflow shows, objects often appear at position (0,0) instead of the specified coordinates top = 191 and left = 271. This happens because the canvas renders objects before properties are fully set.

Relative positioning within groups

According to documentation and research, objects within groups are positioned relative to the group. When you create a group, all internal objects become relative to the group’s coordinates. As explained in this StackOverflow answer:

“They become relative to the group, you can use the group’s left and top to find the absolute position on the canvas”

Deserialization problem

When loading from JSON, a deserialization process occurs where Fabric.js must restore objects and their properties. This process may not correctly handle the group’s positional properties, especially if internal objects have their own coordinates.

Solutions and correct approaches

1. Using callback function correctly

Instead of simply calling canvas.requestRenderAll(), use a more reliable approach with a callback function:

javascript
canvas.loadFromJSON(json, function() {
    // Give time for full loading and property setting
    canvas.renderAll();
    
    // Check group position
    var group = canvas.item(0); // or find group by name
    console.log('Group position:', group.left, group.top);
    
    // Force update
    canvas.requestRenderAll();
});

2. Explicitly specifying group coordinates

When creating a group in PHP, ensure that the group’s coordinates are set correctly:

php
$horizontalCreasing = [
    'type' => 'group',
    "name" => "horizontal_creasing",
    'left' => 0,
    'top' => self::$height,
    'objects' => [],
    // Add required properties for the group
    'originX' => 'left',
    'originY' => 'top',
    'scaleX' => 1,
    'scaleY' => 1,
    'angle' => 0,
];

3. Correct handling of internal objects

Internal objects should have their own coordinates that will be relative to the group:

php
$crease_5 = [
    'type' => 'line',
    'x1' => 0,
    'y1' => 0,
    'x2' => self::$length - self::$thickness,
    'y2' => 0,
    'stroke' => self::$creasingColor,
    'strokeWidth' => 2,
    // Inside the group, these coordinates will be relative
    'left' => 0,  // relative to group
    'top' => 0,   // relative to group
];

4. Using a Fabric.js version with fixes

As mentioned in GitHub issue, version 2.0 had problems with positioning objects within groups. Make sure you’re using the latest stable version of the library.

Example of corrected code

Here’s a complete corrected example:

php
// Creating a group with correct properties
$horizontalCreasing = [
    'type' => 'group',
    "name" => "horizontal_creasing",
    'left' => 0,
    'top' => self::$height,
    'originX' => 'left',
    'originY' => 'top',
    'objects' => [],
];

// Internal object with relative coordinates
$crease_5 = [
    'type' => 'line',
    'x1' => 0,
    'y1' => 0,
    'x2' => self::$length - self::$thickness,
    'y2' => 0,
    'stroke' => self::$creasingColor,
    'strokeWidth' => 2,
    'left' => 0,  // Relative to group
    'top' => 0,   // Relative to group
];
array_push($horizontalCreasing['objects'], $crease_5);
javascript
import * as fabric from 'fabric';

const canvas = new fabric.StaticCanvas('canvas');
const dieline = document.getElementById('canvas').dataset.dieline;
const json = JSON.parse(dieline);

// Use promise-based approach for reliability
canvas.loadFromJSON(json)
    .then(() => {
        // Give time for full initialization
        return new Promise(resolve => setTimeout(resolve, 100));
    })
    .then(() => {
        // Force redraw
        canvas.renderAll();
        
        // Check group position
        const group = canvas.getObjects().find(obj => obj.name === 'horizontal_creasing');
        if (group) {
            console.log('Group position:', group.left, group.top);
        }
        
        // Generate SVG
        return canvas.toSVG();
    })
    .then(svg => {
        console.log('SVG generated successfully');
    })
    .catch(error => {
        console.error('Error loading JSON:', error);
    });

Additional tips

Checking JSON structure

Add debug information to check the JSON structure:

javascript
console.log('JSON structure:', JSON.stringify(json, null, 2));

Using the getObjects method

After loading, check all objects on the canvas:

javascript
canvas.loadFromJSON(json, function() {
    canvas.renderAll();
    const objects = canvas.getObjects();
    objects.forEach((obj, index) => {
        console.log(`Object ${index}:`, obj.type, 'left:', obj.left, 'top:', obj.top);
    });
});

Handling images within groups

If there are images in the group, make sure they are fully loaded before creating the group, as mentioned in the Fabric.js documentation:

javascript
fabric.Image.fromURL('path/to/image.jpg', function(img) {
    img.set({ left: 100, top: 100 });
    var group = new fabric.Group([img], { left: 200, top: 200 });
    canvas.add(group);
});

Conclusion

The problem of group offset when loading from JSON into fabric.js is usually related to:

  1. Timing - properties are not applied immediately after loading
  2. Relative positioning - objects within groups are positioned relative to the group
  3. Incorrect JSON structure - missing required group properties

Recommended actions:

  • Use a promise-based approach for reliable loading
  • Explicitly specify all group properties in PHP
  • Add a delay before final rendering
  • Check the JSON structure and object positions after loading
  • Use the latest stable version of Fabric.js

By following these recommendations, you can correctly manage the positioning of object groups when loading from JSON into fabric.js.

Sources

  1. StackOverflow - Bad position after loading fabric.js JSON
  2. GitHub - Position of elements in a Group
  3. SitePoint - Fabric.js: Advanced
  4. StackOverflow - How to get the canvas-relative position of an object that is in a group
  5. StackOverflow - How does fabric.Group positioning work?
  6. Fabric.js Documentation - Working with groups
How to create a group of objects in fabric.js with correct positioning when loading from PHP?Why do objects inside the group appear in the wrong place after loading from JSON?How to check absolute coordinates of objects inside a fabric.js group after loading?What required properties need to be specified for groups to work correctly in fabric.js when loading from JSON?How to use a promise-based approach for reliable loading of groups in fabric.js from PHP?How to fix the group deserialization issue in fabric.js when working with large data volumes?
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