How many colors png

However, because of the lossy nature of JPG, it is not an ideal way to store art files. Even the highest quality setting for JPG is compressed, and will change the look of your image, if only slightly. JPG is also not an ideal medium for typography, crisp lines, or even photographs with sharp edges, as they are often blurred or smeared out by anti-aliasing. What is potentially worse, is that this loss can accumulate saving multiple versions of artwork can cause degradation with every save.

Even so, it is common to see these things saved as JPG, simply because the file type is so ubiquitous. JPG compression is great if you're just trying to send someone a picture through your phone or in an email, two situations where you might not necessarily want to send large files.

Although most JPGs look fine from a distance under normal compression, there is a noticeable loss of quality whenever users zoom in on a JPG image. The effects of JPG compression have been greatly exaggerated in the image below to show the loss of quality that occurs. And if you were wondering, you can change the extension both ways and the file will continue to work. Unfortunately, JPGs don't support transparency.

JPG files have an unlimited color palette, but they blend pixels together to reduce the size of the image. Compression Image compression is a type of data compression applied to digital images, to reduce their cost for storage or transmission.

There are two types of compression: Lossless Compression Lossy Compression Lossless Compression Lossless means that the image is made smaller, but at no detriment to the quality. Lossy Compression Lossy means the image is made even smaller, but at a detriment to the quality. Colour Depths Colour depth is used to describe the maximum number of colours that are used in the image.

There are also different colour depths palettes : Indexed color Direct color Indexed color Indexed means that the image can only store a limited number of colours normaly , controlled by the author, in something called a Color Map. Therefore, PNG includes many of the benefits of both formats. The alpha channel is added to the three standard color channels red, green, and blue, or RGB and provides levels of transparency.

JPEG images do not support transparent pixels and GIF images only support completely transparent not partially opaque pixels. Therefore, the PNG format allows Web developers and icon designers to fade an image to a transparent background rather than a specific color.

A PNG with an alpha channel can be placed on any color background and maintain its original appearance, even around the edges. While the PNG image format has many benefits, it is not suitable for all purposes. It must be omitted from the new image if any critical chunks have been modified in any way, including the addition of new ones or the reordering or deletion of existing ones. Note that if the program recognizes the chunk, it may choose to modify it appropriately and then copy it to the new file.

Also note that unsafe-to-copy chunks may be copied to the new file if only ancillary chunks have been modified--again, including addition, deletion, and reordering--which implies that ancillary chunks cannot depend on other ancillary chunks. So chunk names encode additional information that is primarily useful if the chunk is not recognized. The remainder of this book will be concerned with known chunks, but before we turn to those, there is one more component of PNG files that has to do with the unknown: the PNG file signature.

As noted earlier, the first 8 bytes of the file are not, strictly speaking, a chunk. But the PNG signature bytes are more than a simple identifier code: they were cleverly designed to allow the most common types of file-transfer corruption to be detected.

The unsuspecting user who transfers a PNG image or other binary file as text is practically guaranteed of destroying it. The 8-byte PNG file signature can detect this sort of problem because it simulates a text file in some respects.

The 8 bytes are given in Table Table PNG Signature Bytes. The first byte is used to detect transmission over a 7-bit channel--for example, email transfer programs often strip the 8th bit, thus changing the PNG signature.

Byte 8 is another Unix end-of-line character. Transfer to or from a Macintosh will strip off the line feeds or replace the carriage return with a line feed, respectively. Either way, the signature is altered, and in all likelihood the remainder of the file is irreversibly damaged. Note that the 9th, 10th, and 11th bytes are guaranteed to be 0 that is, the ASCII NUL character by the fact that the first chunk is required to be IHDR, whose first 4 bytes are its length--a value that is currently 13 and, according to the spec, will never change.

Since NUL bytes are also often stripped out by text-mode transfer protocols, the detection of damaged PNG files is even more robust than the signature alone would suggest.

Before we start putting chunks together, however, a brief interlude on the representation and terminology of color is useful. Color fundamentally refers to a property of light--namely, its wavelength. Each color in the rainbow, from red to purple, is a relatively pure strain of wavelengths of light, and none of these colors can be generated by adding together any of the others. Nor does it end at dull red--smoke on the water glows in the infrared, if only we could see it, and still further down the spectrum are radio waves.

The answer lies not in physics but in physiology. Human eyes contain only three classes of color sensors, which trigger color sensations in the brain in ways that are not yet fully understood. One might guess that these sensors the cones are tuned to red, green, and blue light, but that turns out not to be the case, at least not directly.

Instead, signals from the three types of cones are added and subtracted in various ways, apparently in more than one stage. The details are not especially important; what matters is that the end result is a set of only three signals going into the brain, corresponding to luminosity or brightness , a red-versus-green intensity level, and a yellow-versus-blue level. In addition, the cones are not narrow-band sensors, but instead each responds to a broad range of wavelengths.

The upshot is that the human visual system is relatively poor at analyzing colors, so feeding it different combinations of red, green, and blue light suffices to fool it into thinking it is seeing an entire spectrum. Keep in mind, however, that while true yellow and a combination of red and green may look identical to us, to spectrometers or nonhuman eyes they are quite different. Since they employ pigments, which absorb light rather than emit it, the RGB color space that works so well for computer monitors is inappropriate.

All of these color spaces are beyond the scope of this book, but note that every single one of them has its basis in human physiology. Indeed, if YUV and its brethren sound quite a lot like the set of three signals going into the brain that I just discussed, rest assured that it's not coincidence. Not a single color space in common use today truly represents the full continuum of physical color.

Since black is the preferred color for a huge class of printed material, including text, it is more efficient and considerably cheaper to use a single pigment for it than always to be mixing the other three. Some printing systems actually use five, six, or even seven distinct pigments. Finally, note that image files may represent the appearance of a scene not only as a self-contained item, but also in reference to a background or to other images or text.

In particular, transparency information is often desirable. The simplest approach to transparency in computer graphics is to mark a particular color as transparent, but more complex applications will generally require a completely separate channel of information.

This is known as an alpha channel or sometimes an alpha mask and enables the use of partial transparency, such as is often used in television overlays. PNG adheres to the usual convention that alpha represents opacity; that is, an alpha value of 0 is fully transparent, and the maximum value for the pixel depth is completely opaque. PNG also uses only unassociated alpha, wherein the actual gray or color values are stored unchanged and are only affected by the alpha channel at display time.

The alternative is associated or premultiplied alpha, in which the pixel values are effectively precomposited against a black background; although this allows slightly faster software compositing, it amounts to a lossy transformation of the image data and was therefore rejected in the design of PNG.

IHDR must be the first chunk in a PNG image, and it includes all of the details about the type of the image: its height and width, pixel depth, compression and filtering methods, interlacing method, whether it has an alpha transparency channel, and whether it's a truecolor, grayscale, or colormapped palette image.

Not all combinations of image types are valid, however, and much of the remainder of this chapter will be devoted to a discussion of what is allowed. Figure Layout of the simplest PNG.

IDAT contains all of the image's compressed pixel data. Although single IDATs are perfectly valid as long as they contain no more than 2 gigabytes of compressed data, in most images the compressed data is split into several IDAT chunks for greater robustness.

Most of your images will be in the RGB color model as is our example image , and thus one pixel of your image consists of the three colors or color channels red, green, and blue, each of which can have gradations. And one byte can represent exactly values. With a memory consumption of 3 bytes, an entire pixel can be displayed 1 byte each for red, green and blue.

This is also pretty much the size of an uncompressed BMP image and incidentally the size of a 3. I you wanted to label the color depth, you could speak of a 3-byte image. Since, however, there are color depths other than 1 byte per channel, the designation bit picture that is, 3x8-bit picture has been used for an image, because 1 byte is composed of 8 bits.

The depth of color thus means the memory consumption of a pixel. The two other color depths that you will run across when you are puzzling with the PNG image format are bit and 8-bit:. A bit PNG is similar to a bit PNG, but it also contains an additional 8-bit channel for transparency information usually called alpha channel.

Thus, an image may also include transparent regions in gradations. You have to select "PNG" and mark the checkbox with the name "transparency". An 8-bit PNG works a bit differently. Here, up to colors are defined with their red, green, blue and transparency information in a so-called palette , and for each pixel just the position of the color in the palette will be stored. Thus, to store a pixel, you only need 1 byte of memory for the position of the color in the palette.

Then the saving of the pallete is added. However, with such a picture, you are limited to a maximum of colors, but this is more than enough for the majority of the images for which it makes sense to save them as PNGs.

This is one of the reasons why the first PNG with the balls is smaller than the other. Now we know what color depths mean and how we can calculate the file size of an uncompressed image, but the data still occupies a lot of space and needs to be compressed accordingly. A PNG is compressed in three steps: pre-filtering, dictionary-based coding via LZ77, and finally entropy coding according to a certain Mr. Data can be compressed best if it is repeated as often as possible. So, by pre-filtering, you try to generate the same data from different data.

On each line you could use the filter "Sub" one of 5 possible filters , which means that we simply remember the difference to the previous value. This step can be reversed afterwards, so no data is getting lost.

In our example, it is noted for each line that the filter "Sub" was used. These filters, along with the deflate compression used internally, are the reason why the PNG is a nose ahead of the GIF and therefore almost always smaller. And they provide an important tactic for optimizing the compression of a PNG.

Each PNG algorithm has the ungrateful task of finding out which filter is best for which line in order to repeat as much data as possible. But if it now adapts the filter for line 3 so that the data is similar to those from line 2, it might be that they are no longer similar to those from row 5.

Which solution is the better one?