I used Microsoft Paint to create a 15248 x 6552 solid color picture. I saved it as both .png and .jpg and was expecting the .jpg to be smaller than .png, but it was not.
The .jpg file is 1.49MB, while the .png is 391KB. Shouldnt jpeg being a lossy compression be technically smaller in size?
I read somewhere that .png is better for solid colors etc, so I downloaded a picture form the web (not a solid color) and used paint to save it in both formats. This time the jpeg was smaller than png. Is it solely due to the gradient of colors? if so then why?
Even if the picture is a solid color should jpg encodng be able to compress it even better?
It's to be expected that PNG performs better than JPEG in this scenario.
As pointed out in other answer, PNG does a per-line pixel prediction, followed by ZLIB compression. If the image has a single colour, the prediction will produce a constant zero value for all the pixels, except for the start of each row. Hence the compression will be very effective. I'd bet that if the image were "landscape" (6552 x 15248 instead of 15248 x 6552) the compression would be even a little better.
JPEG compression, instead, divides the image in blocks of 8x8 pixels, and for each one it attempts to quantize finely the low frequency components and coarsely the high frequency components. This works nicely for "natural" (photographic or rendered) images, but no so nicely for images with few colors (or a single one!).
See some comparisons here.
Not necessarily.
PNG is a prediction-based algorithm, which means that it tries to deduct the value of one pixel based on previously coded pixels. I bet the prediction is really accurate for a solid image, thence the very good results.
JPEG accepts different "quality levels" which determine the size of your compressed file. The size differences between your experiment and the web version are likely due to that (unless you're downloading a different image, of course!).
Note that JPEG may introduce some image artifacts because it is a lossy algorithm, while PNG will recover the exact input image for you.
I've found for the same picture that if you save as PNG 1st then JPG the PNG will be smaller and if saved as JPG 1st it will be smaller than the PNG saved afterwards
Ok, so I tried to use the imagemagick command:
"convert picA.png -interlace line picB.png"
to make an interlace version of my .png images. Most of the time, I got the resulting image is larger than the original one, which is kinda normal. However, on certain image, the resulting image size is smaller.
So I just wonder why does that happen? I really don't want my new image to lose any quality because of the command.
Also, is there any compatibility problem with interlaced .png image?
EDIT: I guess my problem is that the original image was not compressed as best as it could be.
The following only applies to the cases where the pixel size is >= 8 bits. I didn't investigate for other cases but I expect similar outcomes.
A content-identical interlaced PNG image file will almost always be greater because of the additional data for filter type descriptions required to handle the passes scanlines. This is what I explained in details in this web page based on the PNG RFC RFC2083.
In short, this is because the sum of the below number of bytes for interlaced filter types description per interlacing pass is almost always greater than the image height (which is the number of filter types for non-interlaced images):
nb_pass1_lines = CEIL(height/8)
nb_pass2_lines = (width>4?CEIL(height/8):0)
nb_pass3_lines = CEIL((height-4)/8)
nb_pass4_lines = (width>2?CEIL(height/4):0)
nb_pass5_lines = CEIL((height-2)/4)
nb_pass6_lines = (width>1?CEIL(height/2):0)
nb_pass7_lines = FLOOR(height/2)
Though, theoretically, it can be that the data entropy/complexity accidentally gets lowered enough by the Adam7 interlacing so that, with the help of filtering, the usually additional space needed for filter types with interlacing may be compensated through the deflate compression used for the PNG format. This would be a particular case to be proven as the entropy/complexity is more likely to increase with interlacing because the image data is made less consistent through the interlacing deconstruction.
I used the word "accidentally" because reducing the data entropy/complexity is not the purpose of the Adam7 interlacing. Its purpose is to allow the progressive loading and display of the image through a passes mechanism. While, reducing the entropy/complexity is the purpose of the filtering for PNG.
I used the word "usually" because, as shown in the explanation web page, for example, a 1 pixel image will be described through the same length of uncompressed data whether interlaced or not. So, in this case, no additional space should be needed.
When it comes to the PNG file size, a lower size for interlaced can be due to:
Different non-pixel encoding related content embedded in the file such as palette (in the case of color type =! 3) and non-critical chunks such as chromaticities, gamma, number of significant bits, default background color, histogram, transparency, physical pixel dimensions, time, text, compressed text. Note that some of those non-pixel encoding related content can lead to different display of the image depending on the software used and the situation.
Different pixel encoding related content (which can change the image quality) such as bit depth, color type (and thus the use of palette or not with color type = 3), image size,... .
Different compression related content such as better filtering choices, accidental lower data entropy/complexity due to interlacing as explained above (theoretical particular case), higher compression level (as you mentioned)
If I had to check whether 2 PNG image files are equivalent pixel wise, I would use the following command in a bash prompt:
diff <( convert non-interlaced.png rgba:- ) <( convert interlaced.png rgba:- )
It should return no difference.
For the compatibility question, if the PNG encoder and PNG decoder implement the mandatory aspects of the PNG RFC, I see no reason for the interlacing to lead to a compatibility issue.
Edit 2018 11 13:
Some experiments based on auto evolved distributed genetic algorithms with niche mechanism (hosted on https://en.oga.jod.li ) are explained here:
https://jod.li/2018/11/13/can-an-interlaced-png-image-be-smaller-than-the-equivalent-non-interlaced-image/
Those experiments show that it is possible for equivalent PNG images to have a smaller size interlaced than non-interlaced. The best images for this are tall, they have a one pixel width and have pixel content that appear random. Though, the shape is not the only important aspect for the interlaced image to be smaller than the non-interlaced image as random cases with the same shape lead to different size differences.
So, yes, some PNG images can be identical pixel wise and for non-pixel related content but have a smaller size interlaced than non-interlaced.
So I just wonder why does that happen?
From section Interlacing and pass extraction of the PNG spec.
Scanlines that do not completely fill an integral number of bytes are padded as defined in 7.2: Scanlines.
NOTE If the reference image contains fewer than five columns or fewer than five rows, some passes will be empty.
I would assume the behavior your experiencing is the result of the Adam7 method requiring additional padding.
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When should certain image file types be used when building websites or interfaces, etc?
What are their points of strength and weakness?
I know that PNG & GIF are lossless, while JPEG is lossy.
But what is the main difference between PNG & GIF?
Why should I prefer one over the other?
What is SVG and when should I use it?
If you don't care about each and every pixel, should you always use JPEG since it's the "lightest" one?
You should be aware of a few key factors...
First, there are two types of compression: Lossless and Lossy.
Lossless means that the image is made smaller, but at no detriment to the quality.
Lossy means the image is made (even) smaller, but at a detriment to the quality. If you saved an image in a Lossy format over and over, the image quality would get progressively worse and worse.
There are also different colour depths (palettes): Indexed color and Direct color.
Indexed means that the image can only store a limited number of colours (usually 256), controlled by the author, in something called a Color Map
Direct means that you can store many thousands of colours that have not been directly chosen by the author
BMP - Lossless / Indexed and Direct
This is an old format. It is Lossless (no image data is lost on save) but there's also little to no compression at all, meaning saving as BMP results in VERY large file sizes. It can have palettes of both Indexed and Direct, but that's a small consolation. The file sizes are so unnecessarily large that nobody ever really uses this format.
Good for: Nothing really. There isn't anything BMP excels at, or isn't done better by other formats.
GIF - Lossless / Indexed only
GIF uses lossless compression, meaning that you can save the image over and over and never lose any data. The file sizes are much smaller than BMP, because good compression is actually used, but it can only store an Indexed palette. This means that for most use cases, there can only be a maximum of 256 different colours in the file. That sounds like quite a small amount, and it is.
GIF images can also be animated and have transparency.
Good for: Logos, line drawings, and other simple images that need to be small. Only really used for websites.
JPEG - Lossy / Direct
JPEGs images were designed to make detailed photographic images as small as possible by removing information that the human eye won't notice. As a result it's a Lossy format, and saving the same file over and over will result in more data being lost over time. It has a palette of thousands of colours and so is great for photographs, but the lossy compression means it's bad for logos and line drawings: Not only will they look fuzzy, but such images will also have a larger file-size compared to GIFs!
Good for: Photographs. Also, gradients.
PNG-8 - Lossless / Indexed
PNG is a newer format, and PNG-8 (the indexed version of PNG) is really a good replacement for GIFs. Sadly, however, it has a few drawbacks: Firstly it cannot support animation like GIF can (well it can, but only Firefox seems to support it, unlike GIF animation which is supported by every browser). Secondly it has some support issues with older browsers like IE6. Thirdly, important software like Photoshop have very poor implementation of the format. (Damn you, Adobe!) PNG-8 can only store 256 colours, like GIFs.
Good for: The main thing that PNG-8 does better than GIFs is having support for Alpha Transparency.
PNG-24 - Lossless / Direct
PNG-24 is a great format that combines Lossless encoding with Direct color (thousands of colours, just like JPEG). It's very much like BMP in that regard, except that PNG actually compresses images, so it results in much smaller files. Unfortunately PNG-24 files will still be bigger than JPEGs (for photos), and GIFs/PNG-8s (for logos and graphics), so you still need to consider if you really want to use one.
Even though PNG-24s allow thousands of colours while having compression, they are not intended to replace JPEG images. A photograph saved as a PNG-24 will likely be at least 5 times larger than a equivalent JPEG image, with very little improvement in visible quality. (Of course, this may be a desirable outcome if you're not concerned about filesize, and want to get the best quality image you can.)
Just like PNG-8, PNG-24 supports alpha-transparency, too.
SVG - Lossless / Vector
A filetype that is currently growing in popularity is SVG, which is different than all the above in that it's a vector file format (the above are all raster). This means that it's actually comprised of lines and curves instead of pixels. When you zoom in on a vector image, you still see a curve or a line. When you zoom in on a raster image, you will see pixels.
For example:
This means SVG is perfect for logos and icons you wish to retain sharpness on Retina screens or at different sizes. It also means a small SVG logo can be used at a much larger (bigger) size without degradation in image quality -- something that would require a separate larger (in terms of filesize) file with raster formats.
SVG file sizes are often tiny, even if they're visually very large, which is great. It's worth bearing in mind, however, that it does depend on the complexity of the shapes used. SVGs require more computing power than raster images because mathematical calculations are involved in drawing the curves and lines. If your logo is especially complicated it could slow down a user's computer, and even have a very large file size. It's important that you simplify your vector shapes as much as possible.
Additionally, SVG files are written in XML, and so can be opened and edited in a text editor(!). This means its values can be manipulated on the fly. For example, you could use JavaScript to change the colour of an SVG icon on a website, much like you would some text (ie. no need for a second image), or even animate them.
In all, they are best for simple flat shapes like logos or graphs.
JPEG is not the lightest for all kinds of images(or even most). Corners and straight lines and plain "fills"(blocks of solid color) will appear blurry or have artifacts in them depending on the compression level. It is a lossy format, and works best for photographs where you can't see artifacts clearly. Straight lines(such as in drawings and comics and such) compress very nicely in PNG and it's lossless. GIF should only be used when you want transparency to work in IE6 or you want animation. GIF only supports a 256 color pallete but is also lossless.
So basically here is a way to decide the image format:
GIF if needs animation or transparency that works on IE6(note, PNG transparency works after IE6)
JPEG if the image is a photograph.
PNG if straight lines as in a comic or other drawing or if a wide color range is needed with transparency(and IE6 is not a factor)
And as commented, if you are unsure of what would qualify, try each format with different compression ratios and weigh the quality and size of the picture and choose which one you think is best. I am only giving rules of thumb.
I usually go with PNG, as it seems to have a few advantages over GIF. There used to be patent restrictions on GIF, but those have expired.
GIFs are suitable for sharp-edged line art (such as logos) with a limited number of colors. This takes advantage of the format's lossless compression, which favors flat areas of uniform color with well defined edges (in contrast to JPEG, which favors smooth gradients and softer images).
GIFs can be used for small animations and low-resolution film clips.
In view of the general limitation on the GIF image palette to 256 colors, it is not usually used as a format for digital photography. Digital photographers use image file formats capable of reproducing a greater range of colors, such as TIFF, RAW or the lossy JPEG, which is more suitable for compressing photographs.
The PNG format is a popular alternative to GIF images since it uses better compression techniques and does not have a limit of 256 colors, but PNGs do not support animations. The MNG and APNG formats, both derived from PNG, support animations, but are not widely used.
JPEG will have poor quality around sharp edges etc. and for this reason it is unsuitable for most web graphics. It excels at photographs.
Compared to GIF, PNG offers better compression, larger pallette and more features, including transparency. And it is lossless.
GIF is limited to 256 colors and do not support real transparency. You should use PNG instead of GIF because it offers better compression and features. PNG is great for small and simple images like logos, icons, etc.
JPEG has better compression with complex images like photos.
As of 2018, we have several new formats, better support for previous formats and some clever hacks of using videos instead of images.
For photographs
jpg - still the most widely supported image format.
webp - New format from google. Good potential, though browser support is not great.
For Icons and graphics
svg - whenever possible. It scales well in retina screens, editable in text editors and customisable via JS/CSS if loaded in DOM.
png - if it involves raster graphics (ie when created in photoshop). Supports transparency which is very essential in this use-case.
For Animations
svg - plus css animations for vector graphics. All advantages of svg + power of css animations.
gif - still the most widely supported animated image format.
mp4 - if animated images are actually short video clips. Twitter / Whatsapp converts gifs to mp4.
apng - decent browser support (i.e. no IE, Edge), but creating it is not as straightforward as gifs.
webp - close to using mp4. Poor support
This is a nice comparison of various animated image formats.
Finally, whichever be the format, make sure to optimize it - There are tools for each format (eg SVGO, Guetzli, OptiPNG etc) and can save considerable bandwidth.
There is a hack that can be done to use GIF images to show true color. One can prepare a GIF animation with 256 color paletted frames with 0 frame delay and set the animation to be shown only once. So, all frames could be shown at the same time. At the end, a true colored GIF image is rendered.
Many software is capable of preparing such GIF images. However, the output file size is larger than a PNG file. It must be used if it is really necessary.
Edit: As #mwfarnley mentioned, there might be hiccups. Still, there are still possible workarounds. One may see a working example here. The final rendered image looks like that:
full-color-gif-image
png has a wider color pallete than gif and gif is properitary while png is not. gif can do animations, what normal-png cannot. png-transparency is only supported by browser roughly more recent than IE6, but there is a Javascript fix for that problem. Both support alpha transparency.
In general I would say that you should use png for most webgraphics while using jpeg for photos, screenshots, or similiar because png compression does not work too good on thoose.
GIF based on a palette of 256 colours per image (at least in its basic incarnation). PNG can do "TrueColour", i.e. 16.7 Million colours out of the box. Lossless PNG compresses better than lossless GIFs. GIF can do "binary" transparency (0% opacity or 100% opacity). PNG can handle alpha transparencies.
All in all, if you don't need to use Alpha-transparent images and support IE6, PNG is probably the better choice when you need pixel-perfect images for vector illustrations and such. JPG is unbeatable for photographs.
Here's an updated answer that includes WebP format:
JPEG:
The JPEG file format was created to optimize photos and other images
that use complex color ranges.
When saving a JPEG (e.g. in Photoshop) you can set the optimization level you want to achieve from lossless meaning no detail is lost to extremely lossy.
In most cases for web applications, you can set the compression to
75% without much losing details.
When to use JPEG? Anytime you have a photo or a graphic with complex color gradients and you can't use webP.
PNG
PNG is primarily a lossless bitmap image format for HQ computer generated images.
Unlike a JPEG, it can have a transparent layer. When you see a transparent image or graphic on the web it's usually a PNG.
When to use PNG? Anytime you have a computer generated graphic or an image with transparency. PNG is not recommended for regular photos as
the file size will generally be significantly larger than the
equivalent JPEG or webP.
GIF:
GIF is a 256 color graphic format supporting both images and animations.
Way back, GIFs were often used for simple graphics and then were slowly replaced by JPEG and PNG.
GIF Images: Low file size and low quality. They have almost no colored depth they only have 256 colors to work with. Replace them with SVGs.
Animated GIF: They can become very large very quickly and can potentially create huge performance issues. Replace them with videos. (Twitter for example, converts all animated GIFs that are uploaded into standard video files and then share those video files instead of the animated GIF.)
When to use GIF? For a web application, just don't! Replace GIF images with SVGs; Replace animated GIFs with videos.
SVG
SVG is a web native graphics format describing lines and curves and shapes and allowing the browser to draw the graphics in real time.
SVGs are scalable, meaning the graphic will look good at any size, all the way from tiny to extremely large.
Even CSS can be applied to them.
When to use SVG? Anytime you have a computer generated graphic
that may need to be scaled or needs to be responsive - like an icon, a logo or a graph.
WebP
A brand new lossless and lossy image format with transparency that was created specifically for the web.
It aims to be a replacement for JPEG as it provides good compression with the addition of transparency.
WebP is supported by all modern browsers but have zero support in older browsers so using them right now requires a fallback using an older image format.
When to use webP? For images and computer graphics if you know your
audience will be using newer browsers. If you need to support older
browsers provide fallbacks to either JPEG or PNG.
The main difference is GIF is patented and a bit more widely supported. PNG is an open specification and alpha transparency is not supported in IE6. Support was improved in IE7, but not completely fixed.
As far as file sizes go, GIF has a smaller default color pallet, so they tend to be smaller file sizes at first glance. PNG files have a larger default pallet, however you can shrink their color pallet so that, when you do, they result in a smaller file size than GIF. The issue again is that this feature isn't as supported in Internet Explorer.
Also, because PNGs can support alpha transparency, they're the only option if you want a variation of transparency other than binary transparency.
If you opt for JPEG, and you're dealing with images for a website, you may want to consider the Google Guetzli perceptual encoder, which is freely available. In my experience, for a fixed quality Guetzli produces smaller files than standard JPEG encoding libraries, while maintaining full compatibility with the JPEG standard (so your images will have the same compatibility as common JPEG images).
The only drawback is that Guetzli takes lot of time to encode.. but this is done only once, when you prepare the image for the website, while the benefits remains forever! Smaller images will take less time to download, so your website speed will increase in the everyday use.
GIF has 8 bit (256 color) palette where PNG as upto 24 bit color palette. So, PNG can support more color and of course the algorithm support compression
As pointed out by #aarjithn, that WebP is a codec for storing photographs.
This is also a codec to store animations (animated image sequence). As of 2020, most mainstream browsers has out of the box support for it (compatibility table).
Note for WIC a plugin is available.
It has advantages over GIF because it is based on a video codec VP8 and has a broader color range than GIF, where GIF limits to 256 colors it expands it to 224 = 16777216 colors, still saving significant amount of space.
I noticed when I saved my menu background image from jpg to gif that gif takes almost one fourth of the size that jpg does.
jpg = 25kb
gif = 7kb
Is there any downsides to using gif?
Its GIF 256 colors BTW. NOT interlaced. Not transparent.
Just want to be sure!
Thanks
If you're not using it for interlacing and transparency, the only real upside would be the compressed file size.
However, you may want to look into using the PNG format. You can find an overview of the three file types here (Surrey University)
Over the past 5 years, PNG has been phased in to the web image standards, developers and designers always knew it handled compression better, but it wasn't fully supported by all browsers - causing it to be overlooked until said browsers were upgraded.
Nowadays, PNG is pretty much the most commonly used file format for images on modern websites.
There are no downsides to this as long as the graphics look good.
For menus and line drawings - in general, any mages with large homogeneous areas, i.e. many adjacent Pixels with the same color -, GIF (or PNG, note musicinmybrain's comment below) is usual the best choice as it compresses stronger, and the results look better (especially because of sharper edges).
For heterogeneous images like photos, GIFalmost always loses in both size, and quality, and JPEG is the better choice.
GIF it's not designed for photographic material, so a big photo comes out better and smaller in JPEG, but tt's probably more than apt for interface elements such as buttons, dividers, borders, etc.
The only caveat that comes to mind are gradients: even on a small area and with 256 colors, GIF will likely show visible banding. This is often very subtle and not a big issue, but you may want to check it, especially if you have lots of gradients and/or hate banding.
I'd second what Daniel May is saying about using the PNG format.
Although if you are solely concerned with image size and don't have a specific reason for using GIF remember that you can tweak the quality of a JPG file to turn a 25kb JPG into a 7kb JPG!
As you say the gif can only handle 256 colors, if you don't need more then that gif will be a good choice.
I would recommend gif over png because there are some issues with color profiles in webbrowsers and pngs. You might end up with png colors not matching the css color.
A few people have mentioned the PNG format. Its worth mentioning there are two different types of PNGs, 8-bit and 24-bit. Both types support transparent pixels, however the 24-bit variant supports alpha transparency.
Generally 8-bit PNGS will be slightly smaller than their GIF equivalents. However large 24-bit PNGs have quite large filesizes if they use alpha-transparencies.
It really all comes down to what is the right format for the image. Some will compress/display better in different formats than others. I wouldn’t try and use one single format, such as GIF for all images and pictures on a website (the original question) unless the images were all IU related.
You really need to understand a little about how these different formats work, rather than choosing one over the other. There is a very good article on the pros & cons of gif/jpg/png formats here:
http://www.websitetemplatedesign.com/oscommerce_tutorials/printer_136.shtml
As far as I am aware, if the image quality looks acceptable then go for the smallest image format you can find. You might find you save even more size using PNG.
To answer your question: NO. It's perfectly acceptable to use GIF files over JPEGs. Each format has its own pros and cons.
You can read about them here: PNG vs. GIF vs. JPEG vs. SVG - When best to use?