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

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

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

  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

    Thanks for this great article. I see it is from 2009 but didn’t stumble upon it until now. We (the company I work for) never really used big sized PNG images in our web projects, but just recently we finished a website which called for large PNG images on top of another background image. Due to the large amount of visitors there is a lot of data traffic on the server which is know casuing problems. I am looking into optimizing all the PNG images and posterization seems like it’s the only way to go since all the images contain shadows. Does anyone know an easy way to apply technique number 2 (Dirty Transparancy) to PNG images containing shadows? I hope someone can help.

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  2. 152

    I used posterize on Paint Shop Pro and set it to 40 as you suggested. It got a huge png from 253kb down to 161kb. I tried a number of the other programs that some suggested here. The best was Riot. It created a gif file with transparency down to 74 kb. However, it was not good enough for professional work. When I used Riot in the png format, it took very little off, and when I went to keep the transparency it actually added to the file size up t0 290 kb.

    I found most of the other programs only take a little off. Very rarely do you find a help as yours which is so much better and so much easier. The posterize trick took 30 seconds. All the rest were time consuming. Great job!

    0
  3. 303

    frankly, i find photoshop an abysmal waste of money and time and got rid of it as PNG is the only raster platform I need for web graphics ~ i feel sorry for people like the author of this article who recognize the vaste superiority of PNG alpha layering over PSD memory rape and mental anguish ~ one of two things could correct this deplorable situation :: Adobe could finally adopt full support for Open Source PNG Alpha Transparency Layering (come on baby, talk to Ulead or Microsoft or Micrographix or or or …) :: unplugged brain drains like this article could disappear forever (that we will not miss)

    -1
  4. 454

    Marwan Salfiti

    July 7, 2011 2:53 pm

    I just used this technique for a few of my websites. I build my sites with WordPress and have been hearing from my clients the the sites were slow. Granted, these are graphic rich sites, but in this day and age, that shouldn’t really matter, right? Wrong! I tried so many tips and tricks and this technique (posterizing the images) worked masterfully. On some images, I crunched more than have the size out.

    To put a number to this, I eliminated over 130k in some instances on a restaurant site where image presentation is key. I posterized, reduced, masked where I could not suffer in quality and tadaaaaa! Thanks for the great tut, one definitely worth saving and referencing over and over.

    0
  5. 605

    Impressive in-depth article on png-optimization.
    A few of these techniques were totaly new to me

    0
  6. 756

    Great article! However there might be something wrong within your post.Check this: “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.””. Should it be ‘SUB’ not ‘add’? And same problem with the “Sub” filter. :)

    0
  7. 907

    I don’t understand how the posterizing thing works. According to the Gimp documentation, a level of 40 means 2^40 colors, while a (no-alpha) PNG is 24-bit, which I would assume would mean 2^24 colors So why does it reduce colors?

    Not that I’m complaining: It works well enough that I got an image to be smaller than a JPEG. Heck, it was almost the size it was when I made it an 8-bit PNG (which looked awful).

    0
  8. 1058

    Jeffrey Faurillo

    August 29, 2011 4:20 pm

    Nice info! now I know how to optimize png files :) kudos!

    0
  9. 1209

    This is a very in-depth article. For developers who want more of a turn-key solution to some of the techniques described here (or even to add a layer of optimization on top of these), try my PNG optimization application for Mac OS X, PNGPress: http://extrafuture.com/code/pngpress/

    0
  10. 1360

    Totally rocking article. I always wondered about Fireworks and Photoshop saving different sized png’s. Those examples are fantastic also, really detailed and teaches me techniques I never knew of.

    0
  11. 1511

    ImageAlpha does these optimisations automatically!

    Especially posterization can be done *much* better than in Photoshop. In the article it reduces 84KB → 53KB, but ImageAlpha can do 37KB on the same image! (or even down to 24KB with PNG8+alpha).

    0
  12. 1662

    Unbeliveble……2. Dirty transparency
    Its really works for me. Thank You

    0
  13. 1813

    Wow, definitely bookmarking this post.

    I can see these techniques being very useful.

    Thanks :)

    1
  14. 1964

    This is a really helpful article. Well done!

    0
  15. 2115

    Very interesting, updated and useful. Also boring actually, that is said by yourself in your article though, but really really a must read for front-end designer.

    0
  16. 2266

    Thank you! Posterise has helped me out amazingly!! All GIMP users – it’s in there under Colours! :)

    0
  17. 2417

    Rolf Timmermans

    June 25, 2012 12:47 am

    We have created an online service at http://tinypng.org that allows you to convert 24-bit PNG files to 8-bit PNG files, while keeping full transparency (semi-transparent pixels are preserved). The number of colours are intelligently reduced, somewhat similar to the posterisation technique described here.

    6
  18. 2719

    Am I missing something on step 2? First of all, I didn’t see “Remove Transparency” anywhere on their website, but I downloaded anyway. When I installed it and restarted my PhotoShop, it doesn’t show up under my filter menu. I’m in PS 6 on Windows.

    0
  19. 3021

    Great article. it’s very useful. Thanks a lot.

    0
  20. 3172

    Great article! Very helpful.
    One more image optimize tool: Lossless Photo Squeezer.
    Give it a try.

    0
  21. 3323

    Herbert van der Wegen

    December 18, 2013 9:03 am

    People! Please just use Color Quantizer – nothing comes close to that little gem for PNG optimization. It’s free, includes an easy to use quality mask brush, and comes with many settings if so desired.

    Batch processing is included, and colour reduction in steps of 16,32,64,128,256,512(!), 1024(!), 2048(!) and 4096(!) is supported as well. The developer is switching to Zopfli soon for even better compression.

    Nothing out there beats it for sheer flexibility in a easy to use GUI.
    http://x128.ho.ua/color-quantizer.html

    1
  22. 3474

    Nice tips.Thanks for sharing..
    One more great tool for Mac to reduce the size of images–IMAGEmini
    More information ,you can search it in Mac App Store.

    0

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