For a personal project I'm creating a static gif decoder. Everything works well so far, except I'm having trouble understanding one concept from the specification:
This block is REQUIRED for an image.
Exactly one Image Descriptor must
be present per image in the Data Stream. An unlimited number of images
may be present per Data Stream.
(Spec)
Unlimited images per data stream? There can be more than 1? I'm assuming they don't mean an animated GIF because in the same specification they state, that they don't recommend gif's be used for animation.
I've read a lot of other sites documenting the gif format and they all state something similar. Heres another link:
There might be an unlimited number of images present in a GIF.
Any ideas?
GIF87a does not allow animation, but enables multiple images in GIF.
The idea was to decompose (for better total compression ratio) large picture into several rectangular subpictures drawn on a common canvas initially filled with background color. Every subpicture has its own x, y, width, height and palette. For example, if you are drawing astronomic picture: Earth and Moon, these two objects may be represented as two small subpictures on common large black canvas. It is not an animation: both subpictures are visible simultaneously.
But this beautiful idea was not supported by software (browsers, picture viewers, etc.), probably due to such non-standard approach.
Since GIF89a these subpictures were transformed into animation frames.
There can be more than one image present in a GIF file. Yes, GIF files can perform crude animation, but it's usually not pretty.
Related
Recently I've been experimenting with spritesheets, particularly in AS3. I've managed to extract standard sprites and develop functional animations through sheets with equal sized squares. A lot of spritesheets on the internet aren't properly organized with equal sized squares though. Is there a way to manipulate pixels to obtain the location and dimensions of sprites from these sheets dynamically? Or must I modify the spritesheet itself to a 'square format'?. I want to avoid hard-coding as much as possible.
The point of spritesheets is often to reduce the overhead of data. Loading one image is much faster than loading hundreds of images. Because you can have variable sized images, most spritesheets generated will also come with a secondary file that defines where in the spritesheet you can find each image. Then, it's just a matter of referencing the x, y, width, and height values for that image.
TexturePacker will generate these, as I imagine most other apps do.
Typically if you have non-regular sized and/or spaced sprites, you'd also have an accompanying data file (XML or JSON probably) that defines the locations and sizes of the different sprites.
I'm working on OCR project but I don't know how to remove graphics from the scanned document image before passing it to tesserract.
Some scanned documents which I want to remove graphics are below:
http://www.mediafire.com/view/hvmpty2z3cw3vao/IMG_0087.JPG
http://www.mediafire.com/view/1sgy5s2aaj2o8y3/IMG_0086.JPG
Any advice is very appreciate. Many thanks.
As the text area is usually sparse and does not connect each other, you may consider to have a sobel edge detection on the original image and detect the biggest connection area with some threshold to detect the image area.
Meanwhile, as the image is a rectangle area, another way is to have a Hough translation to detect straight line to consist a rectangle with 4 lines. If you go this way, it’s recommended that you zoom the image first to reduce the calculate complexity.
You can start by detecting text areas using an algorithm available in AForge.Net. See HorizontalRunLengthSmoothing and VerticalRunLengthSmoothing. The algorithm is not very complicated and you can implement easily it using your favorite image processing library. The only constraint is to know approximately the size of the characters in your images.
I'm developing a visualization for certain parts of a Warehouse with Flex 3. In this visualization there are lot of blocks where 1 to x pallets can be placed where x is between 9 and 15. I need to represent each pallet with a black square, each place which is already assigned to a pallet but not physically taken with a grey square and each free place with a white square. I first thought to just use a canvas for each place on a block and change their color if the state changes. But the hundreds of canvases which are there as a result of this approach are not updated quickly enough for my purposes (screen freezes for a few seconds).
I don't want to use embedded images because of the great amount of images I had to embed in the application (those Images appear in 4 orientations).
My idea was to create background images which reflect the state of the whole block only when needed for that certain state and cache them, so that the computation time is spread over the whole runtime.
My problem now is I don't know how to create them in a way that I can use them as "backgroundImages". As far as I understand I would need them as a class object but I don't know how to achieve that, when not embedding the images.
I'm of course open to better approaches to solve my problem. Thanks for your support.
I would suggest using Graphics property of a Sprite for example. It provides basic drawing API, like drawing lines, circles and rectangles.
Besides, you can draw bitmap images on the Graphics to produce more advances results.
On a website I'm creating, I have about 100 various thumbnails (64x64) that get displayed at different times. On some pages, only 5-15 thumbnails may be displayed. On others, all 100 are loaded.
I'm considering using a technique like CSS sprites to display the images. That is, rather than have image tags for each thumb, do something like:
<span class=thumb1"></span>
And then use CSS to take a slice of one single image with all the thumbs stitched together. That is, one image with all 100 thumbs (in this scenario, a 640x640 image).
My questions:
Is this a good idea?
If yes,
should I do it on all pages the
images occur, or only on the pages
with all the images?
Is there
another technique other than sprites
that may be better than simply
including the images with img tags?
If you think that an ordinary user would visit at least two different pages with these thumbnails than my opinion is that using sprites would really be a good idea!
I would in fact make a single big image with all the thumbnails and then use it in all the pages!
Why?
Fewer http requests (from 100 to 1)! And this is one of the most important thing about web site optimizations (read here from Yahoo Performance Team speed optimization rules )
This way users will download the whole image only the first time and then they will get a super quick loading of that images in all the following pages
The most famous websites on the internet already do that! See sprites used in Yahoo, Amazon or Youtube pages.
You can add other UI or layout images to your sprite
Optimize the resulting PNG:
After you have generated your sprite, if a PNG, you can optimize the PNG even more using this tool: http://www.sitepoint.com/blogs/2009/09/18/squishing-the-last-drops-from-your-pngs/
When not to use sprites:
When part of the images in the sprite change frequently
In this specific case: when the majority of users would need less than the (about) 10% of the images
I'm not quite sure what you mean with "sprites", but this is what I think you mean: instead of 100 images seperately, you create 1 image, with a 10x10 raster. Each coordinate (x,y) contains one of the images you like to show.
Now, if you display an image, you use CSS to set background-location: i.e. x * -64px and y * -64px, perhaps using JavaScript to calculate the x and y for each image-span, or Server-Side scripting to print out a dynamic CSS.
Yes, this is a good idea: it reduces load time, since one only has to download one big image, instead of hundreds of smaller ones.
It depends. If you have caching-abilities for a page, then you can "stitch" all thumbnails into one image file. If you have a very dynamic webpage, then it's quite harsh to build this stitched image every time the thumbnales are updated.
Not sure, if this is what you ment with "sprites", then no, if you ment something else with "sprites", then yes: this answer is an example of one.
It is a good idea if
Speed matters
You don't care about accessibility (screen readers reading the ALT text of an image, etc, all that is gone when you use sprites)
You don't care that your thumbnails are not going to be printed by default in any browser
You can do it without it becoming a maintenance nightmare (which image was on position 461 again?)
As to your second question, it is probably advisable to put all sprites into one or very few container images to minimize loading times.
I don't have much experience with the sprite approach to images (http://www.alistapart.com/articles/sprites). Anyone care to share some pros/cons of sprites vs. old-school slices?
The main advantage of sprites is that the browser has to request less pictures from the webserver. That reduces the number of HTTP requests and makes it possible to compress the parts of the design more effectively. These two points also represent the disadvantages of sliced images.
Here you can see some good examples how sprites improve the loading speed of web pages:
http://css-tricks.com/css-sprites/
Pros:
It's far easier on the server to serve a single large image than many small ones.
It's (slightly) faster for a web browser to load such an image.
Browsers only load images as they needs them - if you are using multiple images in a rollover, the browser would "pause" the first time you roll over the element. This can be solved using sprites, because there is only one image to load.
Cons:
It's kind of a pain to code (more so than using multiple images at least)
One often overlooked downside of using CSS sprites is memory footprint:
https://web.archive.org/web/20130605000516/http://blog.vlad1.com/2009/06/22/to-sprite-or-not-to-sprite/
Unless the sprite image is carefully constructed, you end up with
incredible amounts of wasted space. My favourite example is from WHIT
TV’s web site, where this image is used as a sprite. Note that
this is a 1299×15,000 PNG. It compresses quite well — the actual
download size is around 26K — but browsers don’t render compressed
image data. When this image is downloaded and decompressed, it will
use almost 75MB in memory (1299 * 15000 * 4).
When sprites get loaded into the browser, they are stored uncompressed. A 26 KB file can uncompress to take up a whopping 75 MB of RAM. You should be mindful of using sprites with very large dimensions.
There's also the issue of what happens in browsers with poor CSS support (legacy browsers). The sprites may end up totally broken.
Sprites
Pros:
Less HTTP connections to the server
Faster loading on broadband
Cons:
No encapsulation: If you want to change one image, you have to change the sprite
It is difficult to setup individual images in CSS without a tool
Don't degrade: If the browser don't support CSS, you are in trouble
CSS Sprites:
Pros:
Graceful degrade in unsupported browsers (text can be shown when background images for links are not allowed)
Fewer HTTP requests
Each image has a separate overhead like color table so image slicing will be having more overhead than CSS sprites
Cons:
Poses a problem if images are turned off in the browsers (rare case though)
Image slicing:
Pros:
User perceives a faster load since loaded piece by piece.
Load on demand like when the user places his mouse on the image
Cons:
The webpages might have a large size on the client side even thought it might not be the case on the server side.
The main drawback of sprites is it makes it hard to read/maintain/modify your CSS. It can be difficult to remember the exact pixel offsets within the sprite.
pros using sprites :
since it is using 1 images for all, it require less load on http server.
cons:
- hard to code. you must know the coordinate each images inside sprites so you can display it correctly. once you change the size of the image, you need to adjust all ...
- big images could creates long waited page to display. while using images, user with slow internet connection can see one by one.
best practices.
use it for example roll over images.
Look into using a CSS sprite generator (we use SmartSprites). That way you can do slices locally, and have your build process generate a spritemap. It's the best of both worlds.
Also is SmartSprites isn't for you, there's definitely others, however I like it because it reduces the amount of work up front AND during changes.
Cons
- slower on older browsers/ maybe not working on them with hover effect (opera6)
- if not used correctly can get very/too huge (group them adequately!)
- tedious work to set them up
Pros
- less bytes transfered, because one big image is smaller then all individual images combined (one header/ color table)
- less http requests
I prefer going the middle ground of grouping similar images (normal, hover, selected page, the parent page of selected page) than having all the images in one file. To make these, you image slice like normal in Photoshop or Illustrator, open the files up and combine them with a shortcut key. I wrote the Photoshop script that combines images into CSS sprites. You will have multiple HTTP connections, but won't have the load delay on hover.