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    PNG: Interlaced or not?


    When I save something as a png, Photoshop asks if I want it interlaced or not. What's the difference?
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    If you do not elect to have interlaced then the image will be drawn in a viewers browser from top to bottom, one line at a time - known as progressive.

    Choosing Interlaced means that the whole image is displayed from the get-go, starting at low quality and gradually improving as more data is downloaded to the browser, sort of like coming into focus.

    Obviously the speed of the host server pushing the data and the bandwidth of the users connection has a direct bearing on the time it takes to render the web page and graphics.

    Therefore the option to display the png interlaced or not really depends on the (kb) size of the file and how you would like it rendered to viewers with slower connections.

    When deciding, I compare the file size of png graphics with and without the interlace option and make a decission based on the difference in kb.

    A small image should display quickly across a wide range of speeds and so probably isn't worth adding the extra welly to the file but on a large image (physical size and kb) I would add it because the extra 4k or 5k on a 50kb image is worth it to dislay the whole image from the start so that those on slower connections can at least see that an image is coming in, thus avoiding that 'bugger this, there's no image' click away from your site.

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