Thursday, 7 December 2017

Construct Images JavaScript

We can use single-pixel GIF files to construct images. Because Netscape automatically scales images to the specified width and height, you can create rectangles of various dimensions from a 1 ¥ 1 GIF image of a particular color.
This technique is especially useful for creating dynamic bar charts; but beyond that, its applications are very limited. I won't cover this technique here, but I encourage you to experiment with it on your own.

The greatest drawback to XBM images is that they're monochromatic-in other words, black-and-white, though Netscape renders them as black-and-gray. But this is also something of an advantage to us because they can be represented internally as a string of bits, one per pixel, on or off.
This also makes manipulating them fairly straightforward and not too costly in terms of processor cycles-an important consideration when working with an interpreted language, such as JavaScript. Also important to us, the XBM file's native format is ASCII text, which can be represented using JavaScript strings.

The Animator object let each image load completely before displaying the next image by including an onLoad handler in the BODY tag for each image it writes to the screen.
When the Animator object's toString() method, AnimatorString(), generates a new frame, it sets the ready flag in the object to n, meaning that the Animator is not ready to display a new frame. Once the image has loaded completely, the onLoad handler is called and sets the ready flag back to y.

Loading images from a server has its limitations. Apart from the amount of time this can take-especially over a slow connection-you generally have a fixed set of images to work with.
There are times when it is useful to create images on-the-fly, perhaps in response to user input or to create a dynamic animation.

Images are not immediately available when we want to put them on the screen-they are usually loaded from a server. And there's no reliable way of guessing how long that will take.
If you try to display them on a fixed timetable, most likely none of them would get a chance to load completely: the next image you try to display would clobber the one currently loading, and Netscape would start loading it again from scratch the next time it was called for.

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