Clever PNG Optimization Techniques

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As a web designer you might be already familiar with the PNG image format which offers a full-featured transparency. It’s a lossless, robust, very good replacement of the elder GIF image format. As a Photoshop (or any other image editor) user you might think that there is not that many options for PNG optimization, especially for truecolor PNG’s (PNG-24 in Photoshop), which doesn’t have any. Some of you may even think that this format is “unoptimizable”. Well, in this post we’ll try to debunk this myth.

This post describes some techniques that may help you optimize your PNG-images. These techniques are derived from laborious hours spent on studying how exactly the PNG encoder saves data. We’ll start with some essentials about the PNG format and will then move to advanced optimization techniques.

You may want to take a look at the following related articles:

The boring part

Before we dive into image optimization techniques, we have to learn some technical details about the PNG format. Each graphic format has its own advantages and weaknesses; knowing them will allow you to modify original image for better visual quality and compression. This is a key concept behind professional image optimization.

PNG was developed as an open-source replacement of the proprietary GIF format. They have some common features (like indexed color palette), but PNG is much better than GIF in every aspect. It introduced some cool features for image packing and compression, but for us – web-designers and developers – the most important one is the scanline filtering (also known as ‘delta filters’).

Scanline filtering Link

Here is how it works. For example, we have a 5×5 pixels image with horizontal gradient. Here is a schematic view of this image (each number represents a unique color):

Screenshot

As you can see, all identical colors spread vertically, not horizontally. Such images will have a very poor compression ratio in GIF, because it compresses colors that spread horizontally. Let’s see how this image data can be packed by scanline filtering:

Screenshot

Number 2 before each line represents applied filter, which is “Up” in this case. The “Up” filter sends the message to the PNG decoder: “For the current pixel take the value of the above pixel and add it to the current value.” We have 0 value for lines 2—5 because all pixels in each vertical line have the same color. And such data would be compressed better if the image was relatively large. For example, 15 pixels of value 0 can be written as 0(15) and this is much shorter than fifteen 0’s—this is how compression works in common.

I wrote “can be packed” because in this ideal test case the “Sub” filter (number 1 before each line) will give much better result:

Screenshot

The filter “Sub” sends the message to the decoder: “Take the value of the left pixel and add it to the current value.” In this case, it’s 1. As you may already have guessed, such data will be compressed very effectively.

Scanline filtering is important for us because we can use them: in particular, we can do some image manipulation to make filtering better. There are five filters: None (no filtering), Sub (subtract the left pixel value from the current value), Up (subtract the above pixel value), Average (subtract the average of the left and the upper pixels) and Paeth (substitute the upper, left or upper left pixel value, named after Alan Paeth).

And here’s how these filters affect the image size in comparison with the good ol’ GIF:

Screenshot

GIF, 2568 bytes

Screenshot

PNG, 372 bytes

As you can see, the GIF image is 7 times larger than the same PNG-image.

Image type Link

Another important thing to know about PNG is image type, the meta-data stored inside the file. As a Photoshop user, you are familiar with PNG-8 (indexed image) and PNG-24 (truecolor image). As a Fireworks user, you may know PNG-32 (truecolor with transparency), which is quite confusing, because Photoshop’s PNG-24 may also store truecolor with transparency. Well, it’s worth knowing that these names are not official, and you won’t find them in PNG specs. For your convenience we’ll use Photoshop’s naming convention of PNG image types in this article.

There are 5 available image types in PNG: Grayscale, Truecolor, Indexed-color, Grayscale with alpha and Truecolor with alpha. There are also two subtypes of indexed-color type (non-official, too): bit transparency (each pixel can be fully transparent or fully opaque) and palette transparency (each pixel can be semi-transparent). In second case each color is stored in palette with its alpha value. Thus, opaque red and 50%-transparent red are two different colors and they take 2 cells inside palette.

The worst thing is that Photoshop can save PNG with only 3 of these types: Indexed-color with bit transparency, Truecolor and Truecolor with transparency. That’s why you may find a lot of opinions that Adobe Fireworks is the best tool for PNG optimization. Personally, I don’t agree with them: Fireworks doesn’t have enough tools for image manipulation, it’s only have slightly more options for saving PNG image, but it’s a topic for another discussion.

This is where utilities such as OptiPNG2 or pngcrush3 come in handy. Essentially, these tools do the following:

  1. Pick up the best image type for an image (for example, truecolor can be converted to indexed-color if there aren’t too many colors in the image).
  2. Pick up best delta filters.
  3. Pick up the best compression strategy and, optionally, reduce the color depth.

All these operations do not affect image quality at all, but do reduce the file size of the PNG-images, so I highly recommend you to use such tools every time you save a PNG image.

Now enough with the boring part, let’s do some magic!

1. Posterization

This is a well-known method of the truecolor image optimization. Open up the example image in Photoshop, press the Adjustments layer icon in the Layers palette and choose Posterize:

Screenshot

Pick the smallest possible amount of Levels (usually 40 is enough) and save the image:

Screenshot

Original, 84 KB

Screenshot

Posterized, 53 KB

Here is how it works: the posterization simply reduces the amount of colors, converting similar colors to the single one, thus creating posterized regions. This helps to perform a better scanline filtering and achieve a better compression. The downside of this method is color alternation, which is especially visible if you are trying to stitch image with a HTML background:

Screenshot
Original image

Screenshot
Posterized image

2. Dirty Transparency

Take a look at the following images:

Screenshot
75 KB

Screenshot
30 KB

Both of them were saved in Photoshop without any optimization. Even if you do a per-pixel comparison of these images, you won’t notice any difference. But why is the first image 2.5x larger than the second one?

You need a special plugin for Photoshop to see hidden details. It’s called Remove Transparency and available for free download on the PhotoFreebies plugin suite4. You have to install it first before proceeding with the next step.

Open both images form the example above in Photoshop and choose Filer ? Photo Wiz ? Remove Transparency. Now you can see the actual pixel data that was saved in the image:

Screenshot

Screenshot

What’s happening? How is it possible to reveal the data from the original image from a single-layered PNG image? Well, it’s quite simple. Each pixel in the truecolor image with alpha is described by four bytes: RGBA. The last one is Alpha, which controls pixel transparency: the value of 0 means fully transparent pixel and 255 means fully opaque. And this means that every pixel (with any RGB value) can be hidden with just Alpha byte set to 0. But this RGB data still exists and, moreover, it prevents PNG encoder from effectively packing and encoding the data stream. Thus, we have to remove this hidden data (fill it with solid black, for example) before saving the image. Here is a quick method how to do this:

  1. Open the first image from the example above5 in Photoshop.
  2. Ctrl+click (or ?+click on Mac) on image thumbnail in Layers palette to create a selection, then invert it: Select ? Inverse.

    Screenshot

  3. Switch to Quick Mask mode by pressing Q key:

    Screenshot

  4. We have created a mask for a semi-transparent image, but we need to leave fully transparent pixels only. Choose Image ? Adjustments ? Threshold and move Threshold Level slider to the right, thus leaving fully transparent pixels of the selection:

    Screenshot

  5. Leave Quick Mask mode (press Q key again) and fill the selection with black:

    Screenshot

  6. Invert the seleciton again (Select ? Inverse) and click on the Screenshot icon in the Layers palette to add mask.

That’s it, now you can save this image in PNG-24 and ensure that the 75 KB image is now 30 KB. By the way, all these steps can be easily recorded into Photoshop’s Action (download the Dirty Transparency Photoshop Action6) and reused later with a single keystroke.

You might think about “dirty transparency” as some kind of a bug in image editors: if those image regions can’t be seen and take so much space, why can’t they be removed automatically before saving? Well, this “bug” can be easily turned into a “feature”. Take a look at the following pictures:

Screenshot
5 537 bytes

Screenshot
6 449 bytes

If you remove transparency from those images, you’ll see the following:

Screenshot

Screenshot

Despite the fact that the first image contains more complex image data, it’s 1Kb lighter than the second one, which was optimized as described above. The explanation of this “abnormal” behavior is simple: image data stream in the first example was effectively packed by delta filters, which works better for smooth color transitions (like gradients).

Tech geeks may look at OptiPNG’s output log and ensure that filters were not applied at all for the second image. That’s why I highly recommend you to read The boring part7 of this article first before using these techniques: if you don’t understand what you’re doing, you can make your image even larger.

The ultimate solution to preserve original image data is to create a mask on the image layer in Photoshop (we’ll come back to this later):

Screenshot

As you can see, Dirty transparency is a very powerful yet very delicate technique. You have to know how and why it works before using it. If you are saving PNG-24 images with transparent areas, the first thing you have to do is to check image data in these areas and make the right decision on clearing or leaving them as is.

3. Split by transparency

Sometimes you have to save image in the “heavy” PNG-24 because of few semi-transparent pixels. You can save extra Kbs if you split such images in two parts — one with solid pixels, the second one with semi-transparent — and save them in appropriate graphic formats. For example, you can save semi-transparent pixels in PNG-24, and solid pixels in PNG-8 or even JPEG. Here is a quick (and recordable for Actions) solution to do this. For our experiments we’ll use this elder Russian iPod ancestor:

Screenshot
PNG-24, 62 KB

  1. Ctrl+click/?+click on image thumbnail in Layers palette to create a selection:Screenshot
  2. Go to Channels palette and create new channel from selection:Screenshot
  3. Remove selection (Ctrl+D or ?+D), select the newly created channel and run Threshold (Image ? Adjustments ? Threshold). Move the slider to the very right:Screenshot
  4. We’ve made a mask for selecting dead solid pixels. Now we have to split original layer by this mask. Ctrl+click/?+click on Alpha 1 channel, go to Layers palette, select the original layer and run Layer ? New ? Layer via Cut. As a result, there are two layers with separated solid and semi-transparent pixels.

Now you need to save those two images in separate files: solid pixels in PNG-8, semi-transparent ones in PNG-24. You can apply Posterization8 technique on semi-transparent pixels layer to make image file even smaller.

Screenshot
PNG-8
128 colors + dithering
17 KB

Screenshot
PNG-24
posterization 35
6 KB

And here is the result for comparison:

Screenshot
Before
63 KB

Screenshot
After
23 KB

This method has an obvious drawback: you get two images instead of one, which may be not so convenient to use (for instance, when making a product catalog in the CMS).

4. Influence masks

Actually, is is not a PNG-specific optimization technique, but demonstration of rarely-used Save for Web properties: Color reduction influence mask and Dithering influence mask.

Screenshot

Sadly, these properties were removed in Photoshop CS4, so you can try this optimization approach only in the pre-CS4 versions (I’m using CS3).

To understand how influence masks works, let’s open this demo image9 in PS and save it in PNG-8 with the following settings: Color reduction: Adaptive, Dither: No dithering, Colors: 256.

Screenshot
42 KB

The first thing I’ve noticed about this image is a very fuzzy pendulum. It is a very bright spot on the image and it attracts way too much attention. Let’s try to smooth pendulum’s color transitions by setting dithering to 100%:

Screenshot
46 KB

The pendulum looks better now, but we got another problems: image size increased by 4 KB and solid-color background became very noisy:

Screenshot

We can try to get rid of this noise by lowering dithering value, but the image quality may also be reduced.

Based on these problems, let’s try to do the incredible: increase image quality by lowering the number of colors and image size. Influence masks will help us.

Let’s start with the color. Go to Channels palette, create a new channel and name it color. We’ve already determined that the pendulum is our top priority region to improve image quality, so we need to draw a white circle right on its place (you can enable RGB channel for better precision).

Screenshot

Go to Save for Web dialog and set the following properties: Color reduction: Adaptive, Dither: No, Colors: 128 (as you can see, we reduced number of colors from 256 to 128). Now we have to select an influence mask: click on the Screenshot near Color reduction list and select the color channel from drop-down list: Now our image looks as follows:

Screenshot

You can see the influence mask in action: the pendulum looks perfect, but the other parts of image look really bad. By setting influence mask, we said to Photoshop: “Look, mate, the pendulum is very important part of image so try to preserve as much colors in this area as possible”. Influence mask works exactly the same as regular transparency mask: white color means highest priority in corresponding image region, black color means lowest priority. All intermediate shades of gray affect on image proportionally.

The pendulum now takes the highest color priority, so we have to lower the intensity of white circle to leave more colors for other areas. Close Save for Web dialog, go to Channels palette, select color channel and open Levels dialog (Image ? Adjustments ? Levels). Set the maximum output level to 50 to lower the white color intensity:

Screenshot

Try to save for Web again with the same properties:

Screenshot

Looks better now, but now we’ve got problems in other image areas:

Screenshot

I think you already understand how influence masks works: you provide Photoshop with some clues about important image areas with different shades of gray. With trials and errors I’ve got the following color mask (you can copy it and apply to the image):

Screenshot

Dithering influence mask works exactly the same, but instead of colors, it affects the dithering amount of different image areas. Lighter color means more dithering. This is a very useful feature, because dithering creates irregular pixel patterns which hinders the PNG compressor to use delta filters. You can determine the exact areas where dithering must be applied while leaving other areas intact, thus gaining better compression of image data.

My dithering channel looks like this:

Screenshot

Applying both color and dithering influence channels with the same optimization settings (Adaptive, 128 colors):

Screenshot

Pretty good for 128 colors, isn’t it? Let’s do some finishing touches: set colors to 180 and max dithering to 80%. And here is our final result compared to original, non-optimized version:

Screenshot
256 colors, no dithering, non-optimized
42 KB

Screenshot
180 colors, optimized
34 KB

Please stay tuned (RSS10, Twitter11) for the second part of the article where we’ll cover further techniques; we’ll talk about grayscale images, using less colors, lowering details and discuss further tips for PNG usage and optimization as well as the PNG optimization in practice.

You may want to take a look at the following related articles:


13

Footnotes Link

  1. 1 http://www.smashingmagazine.com/2009/07/01/clever-jpeg-optimization-techniques/
  2. 2 http://optipng.sourceforge.net/
  3. 3 http://pmt.sourceforge.net/pngcrush/
  4. 4 http://www.thepluginsite.com/products/photowiz/photofreebies/index.htm
  5. 5 http://www.smashingmagazine.com/wp-content/uploads/images/png-optimization-guide/dirty-tr-sample1.png
  6. 6 http://www.gracepointafterfive.com/dirty-transparency
  7. 7 #boring
  8. 8 #posterization
  9. 9 http://www.smashingmagazine.com/wp-content/uploads/images/png-optimization-guide/influence/demo.png
  10. 10 http://rss1.smashingmagazine.com/feed/
  11. 11 http://twitter.com/smashingmag
  12. 12 http://www.smashingmagazine.com/2009/07/01/clever-jpeg-optimization-techniques/
  13. 13 http://chikuyonok.ru

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Sergey Chikuyonok is a Russian front-end web-developer and writer with a big passion on optimization: from images and JavaScript effects to working process and time-savings on coding.

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  1. 1

    The “After” image in step 3 is decidedly different than the “Before” image. I assume what I’m supposed to be seeing is the reflection image underneath the opaque one, but it’s not working for me.

    (SM) Sorry, James. There was a mistake in the markup. It is fixed now.

    -2
  2. 2

    DIZZZAMM! Way sweet article. Thanx!

    1
  3. 3

    This is by far the best article Smashing mag have produced this year. I’ve been doing image optimisation for years and thought I knew it all — boy, was I wrong! Some great new techniques to try out here. Many thanks.

    1
  4. 4

    Great article… congratulations

    -1
  5. 5

    this is cool

    0
  6. 6

    sweet!

    0
  7. 7

    Very good article. I never knew about these techniques.

    1
  8. 8

    Excellent article!

    0
  9. 9

    Great, but the “Russian iPod Ancestor” After demo isn’t showing the transparent part of the image in Firefox 3.5 for me… Using OS X, if that helps.

    -1
  10. 10

    VERY interesting post. I was just messing up with Photoshop the other day trying to optimize PNG’s… Thanks!

    0
  11. 11

    great article, keep going!

    0
  12. 12

    Bravo! Nicely done. I’m constantly using PNG and now this just gives me even more of an excuse.

    0
  13. 13

    Wow, thanks for this insightful tutorial. I never knew we could use posterization to reduce the image size without causing much reduction in image quality.

    I especially adore the technique you’ve used for the iPod’s ancestor. Amazing one :)

    0
  14. 14

    I hate using elite speak, but this truly is an “epic” post. I did not know about the dirty png transparency, etc. Thanks so much!

    0
  15. 15

    Excellent article…you guys rock!! thx alot

    0
  16. 16

    great article!
    this might be interesting for osx users
    http://code.google.com/p/imageoptim/

    -1
  17. 17

    Awesome post! All smashing articles should be this good! Share more with us!!

    0
  18. 18

    I wasnt aware of any of these techniques. Thanks a lot.

    -1
  19. 19

    Wow, this was new to me – love it!!!
    Thanks

    0
  20. 20

    Awesome!
    I was just working on png screenshots with a transparent shadow. Out of all these techniques the most precious appears to be the posterization. The other techniques are too complex to bother if they shave only as much size. I’d use them very infrequently on some gem images.
    Thank you so much, the timing of this article couldn’t be more impeccable.

    0
  21. 21

    wow… love it !

    -1
  22. 22

    very nice post

    0
  23. 23

    I LOVE YOU MAN <3

    -1
  24. 24

    Very informative, cool!

    0
  25. 25

    Thank you for a GREAT article. I used to think I was stuck with huge PNG-24 files (I use them a lot for flash ads). This will really help a lot! Fantastic!!

    0
  26. 26

    Excellent!

    0
  27. 27

    Thank you for that article. This information is really appreciated.

    0
  28. 28

    Gracepoint After Five just recently launched punypng (http://www.gracepointafterfive.com/punypng) which combines a lot of the open-source technologies out there. The Ask.com User Experience team that I’m a part of uses punypng as our weapon of choice as well as the Gracepoint Fellowship Church web team. It’s very competitive to smush.it and pngout, and punypng beats them out most of the time (though don’t my word for it, try it out yourself to see).

    I think the great thing about something like punypng versus the command-line tools is that you can throw a whole folder at punypng and just let punypng do all the work to figure out which tool is best for the job. Sometimes converting a JPEG to PNG and then compressing it down is better … sometimes jpeg-tran or jpegoptim is better. Punypng just works.

    For more info about punypng on the Gracepoint After Five blog:
    http://www.gracepointafterfive.com/punypng-making-the-web-a-more-puny-place

    0
  29. 29

    I know the author mentioned that he doesn’t agree with using fireworks due to the lack of manipulation options, but for step #3 if you took that image and saved it with fireworks as a PNG 8 you’d get semi transparent pixels for the whole thing, with one image, and a smaller file size and less HTTP requests (if the output is for the web).

    So please don’t drop Fireworks completely just because its not the best in all cases. It is better than Photoshop for saving png 8 images.

    0
  30. 30

    Loved the article, especially the “boring part.”

    0
  31. 31

    Excellent article. Rocks!

    0
  32. 32

    really great article, and I learned a lot, but the last technique (influence mask) seems to be a bit too time-consuming, to just save 8kB, nowadays where internet is fast enough.

    0
  33. 33

    Sergey Chikuyonok

    July 15, 2009 11:14 am

    So please don’t drop Fireworks completely just because its not the best in all cases. It is better than Photoshop for saving png 8 images.

    True. But Fireworks doesn’t have enough tools for image optimization for PNG.

    but the last technique (influence mask) seems to be a bit too time-consuming, to just save 8kB

    This is just a demonstration how influence masks works. It doesn’t mean that you can save 8 KB only, on other images it might save even more KB.

    0
  34. 34

    ok, so influence mask can save more KBs on bigger images?

    0
  35. 35

    Sergey Chikuyonok

    July 15, 2009 11:29 am

    ok, so influence mask can save more KBs on bigger images?

    It depends on image. It can save more, of course. But I use this technique as a good companion to the “Less colors” method (you’ll see it in the next article) to improve image quality and reduce file size.

    0
  36. 36

    Photoshop really should give us more control over the output of PNGs, it hasn’t changed since ‘save for web’ was first introduced in 5.5…

    Am I right in thinking that Photoshop saves PNGs with extraneous metadata too? OptiPNG/PNGcrush seem to strip this out, plus resolve colour inconsistencies between browsers that I have noticed when you save straight from Photoshop. Would like more info on this if poss.

    0
  37. 37

    Sergey Chikuyonok

    July 15, 2009 11:55 am

    Am I right in thinking that Photoshop saves PNGs with extraneous metadata too?

    No. It saves clean file (if you use Save for Web dialog), but OptiPNG and other tools tries to determine better compression and filtering parameter. OptiPNG also tries to change image type (PNGCrush does not). I’ll give a little more info on this in the next part.

    0
  38. 38

    Re: dirty transparency

    I recorded the Photoshop action as mentioned in the article and made it available for download if anyone wants it.
    http://www.gracepointafterfive.com/dirty-transparency

    (SM) Conrad, thanks. The link was added to the article.

    0
  39. 39

    Wow! Someone buy this guy a beer!

    0
  40. 40

    Thank you for this extremely well written and helpfull article.

    You guys are the best.

    0
  41. 41

    Really, really interesting, thanks.
    Claudio.

    0
  42. 42

    Really good article !!! nice job!!!!!

    0
  43. 43

    Ian Storm Taylor

    July 15, 2009 12:55 pm

    Very interesting article.

    I have a quick question… You describe how PNG encodes vertically while GIF encodes horizontally (sorry if I butchered your information, but you get the point).

    Does this mean that when saving a horizontal gradient we should use GIF? or is PNG more efficient for all gradient directions. (Tried to glean this information from the article, but I couldn’t understand it all.)

    Thanks!

    Please have more articles like this… combined with the JPEG optimization techniques I am now going to completely reoptimize my website and compare load times.

    EDIT: Hmm… after rereading again to try and avoid an unnecessary answer… I think PNG will work just as well for horizontals, because isn’t it the Paeth-type that does this? And it can be used for Up or Left?

    Maybe that’s completely wrong… but let me know.

    0
  44. 44

    The Posterize is a very briljant technique. Thanks alot!

    0
  45. 45

    Am I right in thinking that Photoshop saves PNGs with extraneous metadata too?

    Yes, it does. Any PNG image saved with Photoshop’s Save for Web… or Save As…> PNG includes a tEXt chunk with Software=Imageready. You can see it by opening the image as text, e.g. in a text editor like Notepad++, the text is right there.

    Earlier versions, like Photoshop CS, always added chunks pHYs and cHRM (chromaticities), the latter responsible for color discrepancies in browsers.

    0
  46. 46

    Absolutely brilliant. Bookmarked!

    0
  47. 47

    outstanding

    0
  48. 48

    Sergey,

    Have you done any side-by-side comparisons of some of these techniques with just running the image through OptiPNG or pngcrush? I’m curious what sort of savings the extra manual work might offer.

    0
  49. 49

    So now it’s why PNG compress better than GIF

    Btw how about Animated PNG a.k.a A-PNG, any chance to discuss it?

    0
  50. 50

    Very interesting article — informed me of a lot of things I wasn’t aware of. Not sure I understand it all, but peaked my curiosity anyway. Thanks!

    0

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