It was suggested that this function can be used to retrieve the interlace bit of an image that is stored in a file. This is not the case.
While imageinterlace() returns 0 or 1 if a valid Image resource is passed, passing a file name as a string results in a PHP warning and the return value is neither 0 nor 1.
(PHP 4, PHP 5)
imageinterlace — Ativa ou desativa o entrelaçamento
imageinterlace() muda o bit de entrelaçamento para on ou off.
Se o bit de entrelace for definido e a imagem usada for uma imagem JPEG, a imagem será criada como um JPEG progressivo.
An image resource, returned by one of the image creation functions, such as imagecreatetruecolor().
Se diferente de zero, a imagem será entrelaçada, então o bit de entrelace é desativado.
Retorna 1 se o bit de entrelace é definido para a imagem, 0 caso contrário.
This function is useful when working with Ming, as SWFBitmap constructor will use a NON INTERLACED Jpeg file, so you have to use imageinterlace(0);
dr_snapid's comment that "the server sends every Nth line" is not entirely true. A web server need not know anything about the contents of the file it is sending; its job is simply to send the data. Rather, the image is created in such a way that the data corresponding to "every Nth line" appears at the beginning of the file, with the details becoming able to be filled in as more of the file is received by the browser. In PHP's case, the data may have been generated dynamically instead of pulled from a file, but this does not change the fact that it is the data itself that is different, not the manner in which it is sent.*
In fact, with JPEG, it is less "every Nth line", and more "every Nth pixel", where N is gradually decreased, resulting in a grid that gets progressively more fine-grained as the data is received (hence the appearance of a low res image becoming more detailed). The browser basically estimates what goes in the gaps between pixels, probably by simply blending between the colours, whilst the "real" data continues arriving. This is a fundamentally different method for encoding the data when compared to non-progressive JPEGs, and coupled with the format's other compression techniques, may indeed result in a different file size.
*Can you imagine how much more buggy the Web would be if servers were expected to send different file types using different algorithms, and browsers were expected to be able to receive every one of them?
Interlacing doesnt store another image, it simple changes the order in which the images lines are sent and rendered. The server sends every Nth line, reaches the end, then goes back to the start, reading the lines in between.
After each pass the browser displays the downloaded lines, plus filles the lines not yet received the same, but with each pass the gaps being filled get smaller and the image sharpens. After several passes every line has been read, and the browser has rendered the image in full detail.
Hope that makes sense, it does explain why there should not really be any difference in filesize, so I cant explain why some people have observed a file size difference.
As I understand it, there is only 1 bit in the file which says if its interlaced or not, and the server and client (browser) just handle it differently if it is set to 1.
Set imageinterlace() to 0 if you need to load the generated images in Flash. Flash does not support progressive JPEGs
Interlancing works also with PNG files but it increase filesize (from 14.4M to 17.7M).
just to add my 5 cents on the progressive principle of jpeg: there are no several lowres images stored in jpeg along with original picture, the only thing altered is the order of "pixels". in jpeg the image is divided in areas 8x8 pixels, so instead of linear order of pixels (row-by-row), first there is one pixel form each 8x8 area included in the begining of the image data stream, so when the browser recieves all 8x8 area pixels, it can display "pixelate" image and as soon as it recieves more data, the browser can add more pixels and "sharpen" the image.
About MichaelSoft's note "Imageinterlace($im, 1) creates a JPG which is first loaded completely before showing anything":
Actually, that's not completely true.
This only happens with Internet Explorer (any version, for the time being) as it doesn't seem to support progressive displaying and rather shows the image, when it's 100% done with loading. Other browsers (Mozilla, Mozilla Firefox, Opera, Konqueror, etc.) do their job as they're supposed to do: Displaying a very low-res image, then overlaying a midlow-res image (while loading), and then displaying more and more details.
There is a bug in Microsoft Internet Explorer (at least at present) that means that often, a progressive/interlaced JPEG will actually NOT show at all whilst loading, suddenly appearing only when the entire picture has loaded. A regular NON-interlaced/NON-progressive JPEG will display line by line as it loads, which paradoxically gives the illusion that it's loading faster. MSIE definitely has this one backwards!!
This behaviour is not apparent in other browsers such as Mozilla/FireFox - in these browsers, the image loads progressively, as it should.
Using the interlace-option reduces the filesize! (doesnt really match with the idea that an additional low-res-image is stored)
As this can't return the existing setting for an image, I had to hunt around but found this code to detect if an image has the progressive bit set.
It's not perfect, but it does the job:-