Nonetheless, for web use, that is what we must do. Screen resolutions are not high enough to approach the resolving power of the human eye. Chroma subsampling may be a reasonable compression strategy for high resolution images, but for color web images, one is better served by turning off chroma subsampling and compressing more aggressively to achieve the same file size. The above discussion should be a warning to us when we talk about the quality of JPG compression.
JPG images differ not only in the quality setting of the compression program, but also in the algorithms used by the program authors in achieving that compression. One of the most important algorithmic differences is the the choice of whether or not to use chroma subsampling. Nearly all graphics software employs 2x2 chroma subsampling. Adobe Photoshop switches to 1x1 chroma subsampling at higher quality settings.
Paint Shop Pro offers the best approach, letting the user turn chroma subsampling on or off at any compression setting. One should note that turning off chroma subsampling may not always be the wisest choice for compromising between file size and image quality.
The eye is less sensitive to high spatial frequencies in color information than to high frequencies in intensity. For high resolution images, this means that a good JPG algorithm should store chroma information with less detail for optimum balance between file size and quality. In very special images such as this one, the problem is obvious even unmagnified.
However, choosing 1x1 chroma subsampling means that you must compress intensity information even more to achieve a given file size. For web images which are generally low resolution , turning off chroma subsampling is nearly always a good idea.
For images intended for viewing on a monitor, small file size is best achieved by turning off chroma subsampling and then cranking up the compression. It does not exhibit JPG artifacts of the first image, and it is much sharper than the third image.
Except for the colors of the stripes, this image including its color! The color accuracy of the stripes is far better than the low resolution image compressed with chroma subsampling. Should you use high resolution and high compression or low resolution and low compression to store the best images? You have seen the data and you can draw your own conclusions. This test image is an unusual one chosen because it severely tests chroma subsampling.
Most images will not suffer nearly as much from chroma subsampling as this one. Note that some people wrongly characterize chroma subsampling as reducing color saturation. This is false. Chroma subsampling blurs color detail, which alters colors of narrow stripes that are only a couple of pixels wide.
Higher resolution images retain much image detail even through the considerable compression needed to yield small file sizes. The JPG artifacts degrade the image much less than choosing lower resolution. The two "original" images were both resized from a higher resolution starting image, and saved here as lossless PNG files.
Higher resolution or lower compression JPG's? High resolution Image magnified x 2. Is it just a chroma subsampling problem? Conclusions Should you use high resolution and high compression or low resolution and low compression to store the best images?
JPEGs support bits of color depth or JPEG is actually just a compression algorithm, not a file format. JPEG is designed to exploit certain properties of our eyes, namely, that we are more sensitive to slow changes of brightness and color than we are to rapid changes over a short distance.
While JPEGs are usually the best choice for photographs, on 8-bit monitors they are force-dithered into an 8-bit palette. JPEG compression is treated as 24 bit data 8 bit for gray , regardless of the colors in the original image. Therefore, if you reduce an image from bit to 8-bit prior to JPEG compression, the compression ratio will actually worsen as will the overall quality. JPEG compression introduces noise into solid-color areas, which can distort and even blur flat-color graphics. This is why JPEGs are not well suited to flat-color sharp-edged art or type.
With JPEGs, however, the more you compress, the more edge definition and sharpness you lose. JPEGs do not support transparency, either. It is important to note that saving a graphic to JPEG format with compression should be a last step. Compression effects are cumulative. This means that every time you re-save a JPEG file, you are compressing it further, and thereby tossing away data photographic detail that you can't get back.
Now for the super technical details that explain the prevalence of the RED which is a trick on the eye actually you might want to read this information again emphasis is mine. The starting point of the JPEG compression are the pixels in the primary colors red, green and blue , which are for a lossy compression is not optimally suited. Before the actual compression simply convert the RGB colors, for example, in the YCrCb model that the first channel stores the pure brightness information Y , so the average of the brightness of the red, blue and green channel.
Stores in the second channel is the deviation of the red channel of the average brightness , and in the third channel, the deviation of the blue channel. The value for the green channel can be calculated from this and does not need to be specially recorded.
Once you have separated as components luminance brightness and chrominance color , you can reduce the resolution of the two chrominance channels to half or a quarter, as they for the sharpness does not matter.
The visual cortex of humans contains independent systems for the perception of colors and shapes, and the color-blind would ignore the former fine resolution color boundaries anyway , the color detection system works again with a three to four times as low resolution as the form of recognition.
JPG is a lossy compression method. This means every time you save a jpg image data is thrown away in order to save file size kb. It is important to realize that this loss of data happens each and every time you save a jpg. So if you open a jpg, then save it as a jpg you have thrown away more image data.
It is in areas where the data loss has occurred that artifacts or scum or fuzziness begin to appear. This loss of data is most often noticeable where colors transition from one field of solid color to another field of solid color. There's no direct issue with any particular color specifically. It is more about large areas of similar colors. For images which contain only large areas of flat color, formats such as gif are more appropriate than jpg.
The gif format was designed to maintain large areas of flat color. See, it is not about red color nor blue and green color. It is about the sharp-edges that are visible in YCbCr luma component. JPEG compresses colour equally and therefor does not cause a blur with red, however, the human eye might.
This may be why we see reds blur more than other colours
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