Changing canvas background - javascript

I have HTML5 canvas drawing app and I need to change it's background color when user clicks a button.
Background-color on canvas is working on user-side, but user can save the image to server and I'm exporting canvas data using canvas.toDataURL('image/png'). Problem is, background color of canvas is not included, it only sends data user drawn.
I thought about putting big retangle over whole canvas, but that overwrites all drawings already made. Any ideas?

If you want to add the background after you already have drawn something on the canvas, you could add another canvas with the same size and with "display:none" - then, before sending the image, just fill the temporary canvas with your main canvas element's background color from the CSS, then draw the main canvas onto the temporary one.
Finally, send to server the composite from the temporary canvas rather than the bitmap of the main canvas that has the transparent background.

You're not showing any code, but: You need to draw the background using it's context. Setting the background using CSS will as you already discovered not include it when you extract the image.
Canvas is simply put two parts: the canvas element itself and its bitmap. Using CSS affects the element but not the bitmap. Therefor, you need to update the bitmap usually through its context or using the pixel buffer for it to become a part of the bitmap.
Do the following:
At initialization, draw the background using for example fillRect()
Draw your remaining graphics on top
Extract using toDataURL()

Related

toDataURL() small screens, small saved image

I have created a web application which uses an HTML5 canvas element to allow users to draw in various shapes, colors, line widths. They can also upload an image, have it drawn onto the canvas and then draw on top of that, as a way to annotate their images.
My challenge is that I am using .toDataURL() to get the entire contents of the canvas saved as an image. As I understand, though, this can only capture the size of the canvas. On smaller devices (phones), the image which is saved, ends up quite small dimensions. Here's what I am doing to get what is on the canvas:
var image = document.getElementById("drawingCanvas").toDataURL("image/jpeg", 1.0);
image = image.replace('data:image/jpeg;base64,', '').replace('data:image/png;base64,', '');
Is there some way to set up my canvas to be able to save larger images, even if the size of the canvas is quite small, due to the device?
Sure...
Save the commands used to create the client's drawing.
Create a larger in-memory canvas with document.createElement.
drawImage a full-sized image to the canvas.
context.scale the canvas.
Re-issue the drawing commands.
Export the enlarged canvas with .toDataURL.
Thanks to context.scale you don't have to change any of the coordinates of the drawing commands.
...and alternatively...
You could eliminate step#1 if you're willing to downgrade the drawings to allow some "jaggies". Do this by using CSS to overlay a canvas on top of the client's img. That way the client's drawings are already isolated on the original canvas. Then your procedure simplifies to:
Create a larger in-memory canvas with document.createElement. Make sure the larger canvas has the same aspect ratio as the original canvas.
drawImage a full-sized image to the canvas.
drawImage the original smaller canvas to the larger canvas. You can use the extended version of drawImage to simultaneously scale the client's drawings: context.drawImage(originalCanvas, 0, 0, originalCanvas.width, originalCanvas.height, 0, 0, largerCanvas.width, largerCanvas.height). Yes, the image source for drawImage can be another canvas. ;-)
Export the enlarged canvas with .toDataURL.

Add background pattern to a png image in canvas

As the title says, i want to add background pattern to a png image(not the whole canvas). I tried various guides but they are all about adding background pattern to whole canvas by either using background-image css property or by using canvas's createPattern method.
This is what i'm creating.
As you can see, there are various png images on the canvas(left arm, right arm, body, etc.) and they are all customizable. Now i want to add repeat-pattern to those png images. Those repeat pattern would be an image itself.
How to do that via svg, fabric or any other method?
A really simple way is to create a FOR loop to draw your PNG images at the size you need.
Use the canvas width/height values,your image size and your preferred image distance to determine
the number of iterations your loop needs to run.
On each iteration increment a distance variable a certain value in
order to blit your image at an increasing distance on the x/y axis.
When the loop ends you will have a pattern.

How to re-draw a particular portion of canvas?

I'm creating an HTML 5 game using the <canvas> and JavaScript.
In my canvas I'm loading high resolution images. In each Minute there will come some objects to this canvas. If user clicks on it. It'll be removed from the screen.
My issue:
I'm using clearRect() function for clearing the canvas. And again re-draws all images. Is it necessary that each time I re-draw the entire content ? Is there any option to re-draw that particular area ?
When I used the clearRect() to clear the exact area of that object. The background image also got removed. Is there any way to save the particular portion of background image and draw that portion with this saved image ? (Because my objects are moving on the canvas, I need to retreive the image data dynamically)
I'm new to WebTechnology and HTML 5. So please help me. Thanks in advance
There are a few canvas layer libraries that adds support for the layers:
CanvasLayers
CanvasStack
Think of the canvas as a painting, it is not possible to remove a "layer" of content once it has been applied. You can specify a portion of the canvas to clear using clearRect(x,y,height,width), but you will always need to redraw the background of what you deleted. The drawImage() function takes arguments to draw a specific portion of an image in a specific location, but perhaps more along the lines of what you are looking to use is imageData.
I suggest you do some reading up on the canvas here Mozilla Dev Network: Canvas

Change background image in HTML5 canvas

I need to change background image in HTML5 canvas.
I want to use 2 different thumbnails as buttons and when I click on one of the images the background should be change to that particular image.
Technique 1
On click of the appropriate buttons set a unique CSS class on your <canvas> element.
Use CSS to apply a background image to your canvas.
Drawing commands and erasing your canvas will not affect the CSS-applied background image behind your content.
Technique 2
Write a redraw() function that knows how to erase the canvas and regenerate your drawing, including determining what image to draw as the background first.
On click of the buttons set a variable and re-invoke your redraw() function. It will erase your canvas and use the variable to draw the correct image.

Image inside canvas crop with custom rectangle JQuery

I have a Canvas element with inside an image ( the canvas has the same dimension of the image), i want to crop this image using a selection rectangle! For my project i use the javascript's framework JQuery and i'm searching a plugin that implementing a custom selection rectangle on canvas!
Anyone know one plugin that do this (I founded a lot of plugin that implements this functionality but only on elements)?
If no, what is the way to implement a custom rectangle on Canvas elements?
I hope in yours answers!
did you tried using Fabric.js to generate rectangle on Canvas ?
Usually, everything that is drawn onto a canvas will automatically change the canvas content. If you want to draw a rectangle with the mouse without destroying the underlying canvas, the usual way is to add an additional (temporary) canvas on top of the original canvas. The new canvas has a transparent background, so the underlying canvas is fully visible.
Now you draw the rectangle on the temporary canvas (which has the same dimensions as the origianl) and store the coordinates of the selection to apply the crop process on the original.
Please see my example here: http://jsbin.com/apitak/4

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