Question

I am coding a script in which the user selects a range of data, and then I fetch a bunch of images (over 150) from the server and then I loop trough them to make something like a movie. What I want to know is the most efficient way to load prevent lag when moving trough the images.

Currently I am fetching the images from the server using Ajax and store them in a array of Image objects on the JavaScript. In the HTML I have a div tag in which I wish to put the images. After I finished creating all the Image object (and setting their proper src) in the array, I do the following:

imgElem = document.createElement('img');
document.getElementById('loopLocation').appendChild(imgElem);
imgElem.src = images[0].src;

After this I just make that last call but changing the loop index. I do that every 400ms. The loop works, but sometimes it lags and it freezes on an image for longer that it is supposed to be. I want to know if I am able to improve this anymore from the client side or I just need a server that responds faster.

Answer 1

you might wanna consider spriting which is putting all images into one big image. with this, you only need to load one big image, and then just reposition for every scene.

or, you might also want to pre-load those 150 images, before actually using them. you can use JS array to store Image objects and then loop through that array to get your images.

var images = [];
var expectLoaded = 150;

for(var i = 0; i<expectLoaded;i++){
    (function(i){

        //new image object
        var img = new Image();

        //onload hander
        img.onload = function(){

            //after load, push it into the array
            images.push[img];

            //and check if the all has loaded
            if(images.length === expectLoaded){
                //preload done
            }
        }

        //start loading this image
        img.src = //path to image[i];

    },(i));
}

---

loops block the UI thread. JS is single-threaded, meaning code gets executed in a linear fashion, one after the other. anything that comes after that loop statement will wait until the loop finishes. if that loop takes long… grab some coffee. plus, since you are manipulating the DOM, you don’t see the changes since the UI thread is blocked.

but there are ways to bypass this, and one of them is using timeouts to delay and queue the code for later execution, when JS is not busy.

function animate(frameNo){

    //animate frame[frameNo];

    if(frameNo < total_frames){    //make the UI breate in between frames
        setTimeout(function(){   //so that changes reflect on the screen
            animate(++frameNo);  //then animate next frame
        },200);                  
    }
}

//start
animate(0);