How to Watermark a Picture with Canvas?

How to Watermark a Picture with Canvas?

How to use canvas to watermark pictures in the front end? The following article will introduce you to the method of adding a watermark to a picture using canvas.

How to use canvas to watermark pictures in the front end? The following article will introduce you to the method of adding a watermark to a picture using canvas. There is a certain reference value, friends in need can refer to it, I hope to help everyone.

Adding a watermark to a webpage image in the front end is a common feature and one of the ways that Internet content authors protect their copyrights. This article briefly records the implementation of watermarking on the front end using canvas.

The canvas element is actually a canvas. We can easily draw some text, lines, graphics, etc. It can also draw an image rendered in an img tag on the canvas.

When uploading a file to the backend, after reading the user's local file using the input tag, it is actually a Blob object (more precisely, a File object, a special Blob object); and an image is displayed on the page using img Labels; drawing functions are implemented with canvas. The function of adding watermarks needs to convert between the img tag, canvas, and Blob objects. This can be done through some APIs:

We can read the image blob locally, then render it to the img tag, use canvas to draw the img content and draw the watermark content to the canvas, and then convert the canvas content to a Blob object upload server, so that the function of image + watermark is fully implemented.


First, read the image file locally and render it to the img tag

Reading the image file locally will get a Blob object. We can use the FileReader.readAsDataURL method to read the content of the Blob and get a Base64 encoded file content. This content can be assigned to img.src to be rendered on the browser. Local image. Of course, img does not have to be rendered into the DOM tree. The read operation is an asynchronous operation. The load event is triggered when the read is completed. In order to facilitate subsequent calls, we can wrap this operation with a Promise and finally return a Promise object.

function blobToImg (blob) {
  return new Promise ((resolve, reject) => {
    let reader = new FileReader ()
    reader.addEventListener ('load', () => {
      let img = new Image ()
      img.src = reader.result
      img.addEventListener ('load', () => resolve (img))
    })
    reader.readAsDataURL (blob)
  })
}

Second, draw the img tag content to the canvas canvas

Calling the drawImage method of the canvas context object of the canvas element can be used to draw the img content to the canvas.

function imgToCanvas (img) {
  let canvas = document.createElement ('canvas')
  canvas.width = img.width
  canvas.height = img.height
  let ctx = canvas.getContext ('2d')
  ctx.drawImage (img, 0, 0)
  return canvas
}

The drawImage method can pass multiple parameters to define the range of the image to be drawn. Here, 0, 0 is defined to start drawing from the upper left corner of the image, and two parameters can be passed to define the end point of the image drawing, but here The entire picture is drawn to the canvas, so the default value is sufficient.


Third, draw a watermark on the canvas and convert it to a Blob object

When uploading images, we usually use FormData, and the image file is put into FormData as a Blob object, so we need to convert the canvas to Blob for file upload and other operations. Using the HTMLCanvasElement.toBlob method:

function watermark (canvas, text) {
  return new Promise ((resolve, reject) => {
    let ctx = canvas.getContext ('2d')
    // Set fill size and font, style
    ctx.font = "24px Times New Roman"
    ctx.fillStyle = "# FFC82C"
    // Set right alignment
    ctx.textAlign = 'right'
    // Draw text at the specified position, here specify the location from the bottom right corner 20 coordinates
    ctx.fillText (text, canvas.width-20, canvas.height-20)
    canvas.toBlob (blob => resolve (blob))
  })
}

Fourth, a complete example of watermarking in pictures

The above three steps are combined to complete the watermarking from the picture. The following is a simple use example: select an image file locally, then add the watermark, and preview the added watermark under the incoming dom element image.

function imgWatermark (dom, text) {
  let input = document.createElement ('input')
  input.setAttribute ('type', 'file')
  input.setAttribute ('accept', 'image / *')
  input.onchange = async () => {
    let img = await blobToImg (input.files [0])
    let canvas = imgToCanvas (img)
    let blob = await watermark (canvas, text)
    // Here read the blob to the img tag and render it in the dom; if it is an upload file, you can add the blob to FormData
    let newImage = await blobToImg (blob)
    dom.appendChild (newImage)
  }
  input.click ()
}

Add a div element with the container id to the page and call it as follows:

let dom = document.querySelector ('# container')
imgWatermark (dom, 'Watermark text')

This completely adds a watermark effect to the picture. Let's take a look at the actual effect below, and you can also experience it online .

Before adding a watermark:

After adding a watermark (watermark content: "morioh.com"):

Five. Summary

This article only introduces the simple implementation of image + watermark text, only some examples are introduced, in fact, more complex watermark effects can be achieved, such as images, translucent or gradient text, but the principle is the same. In addition, some of the interfaces involved in this article are actually very useful. For example, a common function is preview and crop of avatar upload. At this time, you can use FileReader to read the preview of the file content, and use CanvasRenderingContext2D.drawImage to implement the crop function. For more detailed usage of the interfaces involved in this article, you can refer to the MDN documentation. The APIs in this article use the form of links. You can quickly view their documentation.

Understanding of Meter Tag in HTML 5

Understanding of Meter Tag in HTML 5

This post explains the HTML 5 meter tag and how to create it

What is the meter tag?

I this article I am explaining the meter tag. The output of both (progress and meter) tags is the same as we will see but there is the difference that actually the meter tag is used to represent a scalar measurement within a known range. The value can be fractional.Examples

Disk uses, the relevance of a query result, the fraction of a voting population to have selected a specific candidate.

What is the difference between the progress tag and the meter tag? Progress bar is used to display the progress of a specific task. Or a progress element represents the completion progress of a task. Whereas the meter tag is used to represent guages.  We can think that a progress tag represents dynamic data whereas a meter tag represents static data.

Note:

  1. According to the W3C, the meter element should not be used to indicate progress, because to indicate the progress we have the progress tag.
  2. The meter element also does not represent a scalar value of an arbitrary range; for example, it would be wrong to use this to report a weight, or height, unless there is a known maximum value.

Syntax

The Meter tag is an inline element, the same as a header, progress and so on.

Attributes

<meter></meter>   

Apart from the Global Attributes and Event Attributes, the meter tag have 6 more attributes as shown in the following table:

The following inequalities must hold, as applicable:

  • min <= value <= max
  • min <= low <= max (if low is specified)
  • min <= high <= max (if high is specified)
  • min <= optimum <= max (if optimum is specified)
  • low <= high (if both low and high are specified)

Note: if you do not specify min or max then the range will be between 0.0 to 1.0 and the value must be in that range.

HTML DOM Meter Object

In the DOM the meter element is defined by METER that represents for HTML 5.

  • How the meter object can be accessed: using the getElementById() method you can access the element.
var x = document.getElementById("[Give id of the meter tag here]");  
  • Create a meter object: You can also create the element dynamically using the createElement() method:
var x = document.createElement("METER");    
  • Meter object properties: The properties of a meter object are given in the following table:

Example 1: This example will show you how can you use a meter element in HTML 5.

<!DOCTYPE html>        
<html lang="en">        
<head>        
    <meta charset="utf-8" />        
</head>        
<body>        
   <b>Meter without value</b>    
   <meter></meter>    
    
   <br/><br/>    
   <b>Meter with value but without min and max attribute</b>    
   <meter value="0.8"></meter>    
    
   <br/><br/>    
   <b>Meter with value, min and max attribute</b>    
   <meter min="0" max="100" value="17"></meter>    
    
   <br/><br/>    
   <b>Meter (when "min <= value < low")</b>    
   <meter  min="0" max="100" value="17" low="25" high="75"></meter>    
    
   <br/><br/>    
   <b>Meter (when "high < value <= max")</b>    
   <meter  min="0" max="100" value="80" low="25" high="75"></meter>    
    
   <br/><br/>    
   <b>Meter (when "low <= value <= high")</b>    
   <meter  min="0" max="100" value="50" low="25" high="75"></meter>    
    
   <br/><br/>    
   <b>Meter with optimum attribute</b>    
   <meter  min="0" max="100" value="24" low="25" high="75" optimum="80"></meter>    
    
   <br/><br/>    
   <b>Meter with optimum attribute</b>    
   <meter  min="0" max="100" value="80" low="25" high="75" optimum="20"></meter>    
    
</body>        
</html>     

The output of example 1:

Styling meter element: we can define various types of styles for the meter element using a progress selector.Example 2: This example will explain how to increase the width and height of a meter element.

Example 2: This example will explain how to increase the width and height of a meter element.

<!DOCTYPE html>        
<html lang="en">        
<head>        
    <meta charset="utf-8" />     
   <style>    
   meter {    
     width: 400px;    
     height: 25px;    
   }    
   </style>       
</head>        
<body>        
   <b>Meter without value</b>    
   <meter></meter>    
    
   <br/><br/>    
   <b>Meter with value but without min and max attribute</b>    
   <meter value="0.8"></meter>    
    
   <br/><br/>    
   <b>Meter with value, min and max attribute</b>    
   <meter min="0" max="100" value="17"></meter>    
       
   <br/><br/>    
   <b>Meter (when "min <= value < low")</b>    
   <meter  min="0" max="100" value="17" low="25" high="75"></meter>    
    
   <br/><br/>    
   <b>Meter (when "high < value <= max")</b>    
   <meter  min="0" max="100" value="80" low="25" high="75"></meter>    
    
   <br/><br/>    
   <b>Meter (when "low <= value <= high")</b>    
   <meter  min="0" max="100" value="50" low="25" high="75"></meter>    
    
   <br/><br/>    
   <b>Meter with optimum attribute</b>    
   <meter  min="0" max="100" value="24" low="25" high="75" optimum="80"></meter>    
    
   <br/><br/>    
   <b>Meter with optimum attribute</b>    
   <meter  min="0" max="100" value="80" low="25" high="75" optimum="20"></meter>    
    
</body>        
</html>     

Output of the Example 2:

More in styling:

  • WebKit/Blink Browser: like Opera, Googe Chrome and Safari
  • FireFox
  • Internet Explorer

1. WebKit/Blink Browser: According to webkit.org, we can have 5 different pseudo classes that are given in the following table.

Example 3:  This example explains how to reset the appearance of a meter element. To reset the appearance we use -webkit-appearance:none;

<!DOCTYPE html>          
<html lang="en">          
<head>          
    <meta charset="utf-8" />       
<style>      
meter {      
  width: 300px;      
  height: 25px;      
  -webkit-appearance: none; /* Reset appearance */      
  border: 1px solid #ccc;      
  border-radius: 5px;      
}      
</style>         
</head>          
<body>          
<b>Meter without value</b>      
<meter></meter>      
      
<br/><br/>      
<b>Meter with value but without min and max attribute</b>      
<meter value="0.8"></meter>      
      
<br/><br/>      
<b>Meter with value, min and max attribute</b>      
<meter min="0" max="100" value="17"></meter>      
      
<br/><br/>      
<b>Meter (when "min <= value < low")</b>      
<meter  min="0" max="100" value="17" low="25" high="75"></meter>      
      
<br/><br/>      
<b>Meter (when "high < value <= max")</b>      
<meter  min="0" max="100" value="80" low="25" high="75"></meter>      
      
<br/><br/>      
<b>Meter (when "low <= value <= high")</b>      
<meter  min="0" max="100" value="50" low="25" high="75"></meter>      
      
<br/><br/>      
<b>Meter with optimum attribute</b>      
<meter  min="0" max="100" value="24" low="25" high="75" optimum="80"></meter>      
      
<br/><br/>      
<b>Meter with optimum attribute</b>      
<meter  min="0" max="100" value="80" low="25" high="75" optimum="20"></meter>      
      
</body>          
</html>      

Output of the Example 3:

Example 4: How to render the background container?

According to the preceding table I have described that we can render the container using the -webkit-meter-bar pseudo-class, so to apply the rendering I am using the following code:

meter::-webkit-meter-bar {    
  background: none;     
  background-color: whiteSmoke;    
  box-shadow: 0 5px 5px -5px #00F inset;    
  border: 1px solid #0ff;    
  border-radius: 5px;    
}  

Complete Code

<!DOCTYPE html>        
<html lang="en">        
<head>        
    <meta charset="utf-8" />     
<style>    
meter {    
  width: 300px;    
  height: 25px;    
  -webkit-appearance: none; /* Reset appearance */    
  border: 1px solid #F0f;    
  border-radius: 5px;    
}    
meter::-webkit-meter-bar {    
  background: none;     
  background-color: whiteSmoke;    
  box-shadow: 0 5px 5px -5px #00F inset;    
  border: 1px solid #0ff;    
  border-radius: 5px;    
}    
    
</style>       
</head>        
<body>        
<b>Meter without value</b>    
<meter></meter>    
    
<br/><br/>    
<b>Meter with value but without min and max attribute</b>    
<meter value="0.8"></meter>    
    
<br/><br/>    
<b>Meter with value, min and max attribute</b>    
<meter min="0" max="100" value="17"></meter>    
    
<br/><br/>    
<b>Meter (when "min <= value < low")</b>    
<meter  min="0" max="100" value="17" low="25" high="75"></meter>    
    
<br/><br/>    
<b>Meter (when "high < value <= max")</b>    
<meter  min="0" max="100" value="80" low="25" high="75"></meter>    
    
<br/><br/>    
<b>Meter (when "low <= value <= high")</b>    
<meter  min="0" max="100" value="50" low="25" high="75"></meter>    
    
<br/><br/>    
<b>Meter with optimum attribute</b>    
<meter  min="0" max="100" value="24" low="25" high="75" optimum="80"></meter>    
    
<br/><br/>    
<b>Meter with optimum attribute</b>    
<meter  min="0" max="100" value="80" low="25" high="75" optimum="20"></meter>    
    
</body>        
</html>     

Output of the Example 4:

Example 5: This example explains how to render the value of the meter element, it means inside the container how to apply the rendering.

According to the table we can render the meter elements using the following pseudo classes:

-webkit-meter-optimum-value  
-webkit-meter-suboptimum-value  
-webkit-meter-even-less-good-value  

Code: I am using the following code:

  1. For -webkit-meter-optimum-value:
meter::-webkit-meter-optimum-value {    
  box-shadow: 0 5px 5px -5px #999 inset;    
  background-image: linear-gradient(    
    25deg,     
    #002900 5%,     
    #003D00 5%,    
    #005200 25%,    
    #007A00 25%,    
    #00A300 55%,    
    #00CC00 55%,    
    #33D633 95%,    
    #66E066 95%,    
    #99EB99 100%    
  );    
  background-size: 100% 100%;    
}   
  1. For -webkit-meter-suboptimum-value:
meter::-webkit-meter-suboptimum-value {    
  box-shadow: 0 5px 5px -5px #999 inset;    
  background-image: linear-gradient(    
    25deg,     
    #333300 5%,     
    #666600 5%,    
    #999900 25%,    
    #CCCC00 25%,    
    #FFFF00 55%,    
    #FFFF33 55%,    
    #FFFFCC 95%,    
    #FF3300 95%,    
    #B22400 100%    
  );    
  background-size: 100% 100%;    
}   
  1. For -webkit-meter-even-less-good-value:
meter::-webkit-meter-even-less-good-value  {    
  box-shadow: 0 5px 5px -5px #999 inset;    
  background-image: linear-gradient(    
    25deg,     
    #000000 5%,     
    #330000 5%,    
    #660000 25%,    
    #990000 25%,    
    #CC0000 55%,    
    #FF0000 55%,    
    #FF3333 95%,    
    #FF6666 95%,    
    #FF9999 100%    
  );    
  background-size: 100% 100%;    
}   

Complete Code:

<!DOCTYPE html>        
<html lang="en">  
   <head>  
      <meta charset="utf-8" />  
      <style>    
         meter {    
         width: 300px;    
         height: 25px;    
         -webkit-appearance: none; /* Reset appearance */    
         border: 1px solid #F0f;    
         border-radius: 5px;    
         }    
         meter::-webkit-meter-bar {    
         background: none;     
         background-color: whiteSmoke;    
         box-shadow: 0 5px 5px -5px #00F inset;    
         border: 1px solid #0ff;    
         border-radius: 5px;    
         }    
         meter::-webkit-meter-optimum-value {    
         box-shadow: 0 5px 5px -5px #999 inset;    
         background-image: linear-gradient(    
         25deg,     
         #002900 5%,     
         #003D00 5%,    
         #005200 25%,    
         #007A00 25%,    
         #00A300 55%,    
         #00CC00 55%,    
         #33D633 95%,    
         #66E066 95%,    
         #99EB99 100%    
         );    
         background-size: 100% 100%;    
         }    
         meter::-webkit-meter-suboptimum-value {    
         box-shadow: 0 5px 5px -5px #999 inset;    
         background-image: linear-gradient(    
         25deg,     
         #333300 5%,     
         #666600 5%,    
         #999900 25%,    
         #CCCC00 25%,    
         #FFFF00 55%,    
         #FFFF33 55%,    
         #FFFFCC 95%,    
         #FF3300 95%,    
         #B22400 100%    
         );    
         background-size: 100% 100%;    
         }    
         meter::-webkit-meter-even-less-good-value  {    
         box-shadow: 0 5px 5px -5px #999 inset;    
         background-image: linear-gradient(    
         25deg,     
         #000000 5%,     
         #330000 5%,    
         #660000 25%,    
         #990000 25%,    
         #CC0000 55%,    
         #FF0000 55%,    
         #FF3333 95%,    
         #FF6666 95%,    
         #FF9999 100%    
         );    
         background-size: 100% 100%;    
         }    
      </style>  
   </head>  
   <body>        
      <b>Meter without value</b>    
      <meter></meter>    
      <br/><br/>    
      <b>Meter with value but without min and max attribute</b>    
      <meter value="0.8"></meter>    
      <br/><br/>    
      <b>Meter with value, min and max attribute</b>    
      <meter min="0" max="100" value="17"></meter>    
      <br/><br/>    
      <b>Meter (when "min <= value < low")</b>    
      <meter  min="0" max="100" value="17" low="25" high="75"></meter>    
      <br/><br/>    
      <b>Meter (when "high < value <= max")</b>    
      <meter  min="0" max="100" value="80" low="25" high="75"></meter>    
      <br/><br/>    
      <b>Meter (when "low <= value <= high")</b>    
      <meter  min="0" max="100" value="50" low="25" high="75"></meter>    
      <br/><br/>    
      <b>Meter with optimum attribute</b>    
      <meter  min="0" max="100" value="24" low="25" high="75" optimum="80"></meter>    
      <br/><br/>    
      <b>Meter with optimum attribute</b>    
      <meter  min="0" max="100" value="80" low="25" high="75" optimum="20"></meter>    
   </body>  
</html> 

Output of the Example 5:

2. FireFox Browser: For the FireFox browser use "-moz" instead of "-webkit" and everything else is the same.

3. Internet Explorer: In IE you can use all these properties directly, like appearance.

Thank you for reading ! I hope this tutorial will surely help!

Learn HTML5 Canvas - DRAW - HTML & JavaScript

Learn HTML5 Canvas - DRAW - HTML & JavaScript

We go over the basics of the HTML5 Canvas element and use JavaScript to draw lines, circles, squares and rectangles to the DOMs Canvas. JavaScript combined with HTML5 Canvas can be used to draw elements anywhere you want inside of the HTML5 Canvas. We learn the basics in a simple how to video but in the next part we will look how to animate the HTML5 Canvas using JavaScript to change the X & Y coordinates of shapes we create. We will follow an Object Oriented Programming pattern.

The Ultimate Guide to Web Development with HTML and CSS

The Ultimate Guide to Web Development with HTML and CSS

The Ultimate Guide to Web Development with HTML and CSS. Learn Web Development by creating a Blog with HTML and CSS. Make a web applications using HTML5 and CSS3. Create a blog from Scratch.

The Ultimate Guide to Web Development with HTML and CSS

The Ultimate Guide to Web Development with HTML and CSS. Learn Web Development by creating a Blog with HTML and CSS. Make a web applications using HTML5 and CSS3. Create a blog from Scratch.

If you want to create your own website or want to become a web developer, you are in the right place. This course, you are going to learn about html language which is the most important skill you must know in web developer career.

In addition, html is used to create website skeleton, to make them look nice, you must apply CSS style to them. With the latest technology, you will be able to make a web-page layout and structure according to what you want or what your client want.

Honestly, There are many courses available on the internet include free and fee. how ever, most of them are really hard to follow along. That is the reason that I create this web development course on Udemy which is easier to understand. Do you want to spend a month to learn or just spend less time but get the concept and have ability to develop a website? When you know its concept, you will no longer need any teacher. I mean you can learn and research more by yourself.

What you'll learn

  • Make a web applications using HTML5 and CSS3
  • Create a blog from stretch
  • Understand and have skill how website work
  • Be a comfortable front-end developer