So I just wanted to ask this question because there is a quality slider for png in Adobe Photoshop.
Now the reason why I find this very confusing is I know that PNG is a lossless format.
I'm not sure what is used to measure objective quality, by that I mean how much information is contained in images. I'm assuming it's either dpi or bit depth or file size for the same format?
Now, I know that the same lossless compression for the same image can have varying quality (bit depth and dpi).
This is because you can do an in between step and encode it into a lossy format like jpg, before converting back into a lossless format like png.
For example, we have a picture of some dice taken by a camera in a lossless png format it's the original quality let's say 360dpi horizontal and vertical resolution and 24 bit depth.
Then we can encode it into jpg (which is using lossy compression) and presumably the quality would go down slightly from averaging pixels, as it is lossy compression and it would lower the quality to something like 200dpi horizontal and vertical resolution and 12 bit depth.
Then we encode that back into png again (lossless compression) and the final quality would be 200dpi horizontal and vertical resolution and 12 bit depth.
So now we have two pngs for the same image which are in a lossless format, the original one and our one that was encoded from a jpg, that have different quality.
The point I'm trying to make is that I think this is why two images using lossless compression can have differing quality, even though lossless compression does not remove any information, because one of the lossless compression images could have just been encoded from a lossy format. Information can be removed from lossless formats by encoding to and from a lossy format.
So now back to the question, is this why Photoshop has a quality slider for PNG images?
For qualities below 100% say 80%, is Photoshop encoding into something lossy like jpg and then back into the lossless png so that the files can have different quality and thus, a different file size for the same image?
I've also read something on wikipedia about PNG being able to perform lossy compression as well, so is that what is happening instead?
https://en.wikipedia.org/wiki/Portable_Network_Graphics#Lossy_PNG_compression
I'm just confused about it.
Photoshop may be reducing color depth as described in the the wiki you included in your question:
reducing color depth, either:
use a palette (instead of RGB) if the image has 256 or fewer colors,
use a smaller palette, if the image has 2, 4, or 16 colors, or
(optionally) lossily discard some of the data in the original image
Related
We all know that converting from jpg to jpg makes an image lose quality, and that the same doesn't happen when it's png to png, but what about jpg to png ?
In my mind it makes sense that it doesn't, but still I'm not sure.
I think that good answer to this question probably would be an article in which there would be a good explanation of why it happens or not. Do you now of any ? I couldn't find such article.
It doesn't, but it's complicated.
Reading of JPEG is not a precisely defined process. Different JPEG decoders are allowed to produce slightly different results from the same file.
Converting JPEG to PNG makes pixels forever represent the particular method that was used to decode the JPEG, even if it wasn't the best one. If you use a "bad" JPEG decoder for the conversion you lose ability to use a "better" JPEG decoder later.
The differences are:
Chroma upsampling. JPEG may store chroma (~color) at lower resolution than luma (~brightness) of the image. The spec doesn't say how the chroma channel should be resized, so some decoders use blocky nearest-neighbor scaling, some use bilinear scaling, some do even weirder things.
Numeric precision. JPEG requires calculations to be done to convert image from DCT and YCbCr representation to RGB. This can be done quickly and cheaply using integer math and 8-bit color, or a tiny bit better using floating-point math and higher-depth color.
Color profiles and CMYK. Conversion may change color space (in case of CMYK it has to convert to RGB), which is easy to get wrong.
There are other reasons to avoid the conversion:
The file will almost certainly become much larger. JPEG compression artifacts are the worst case for PNG.
The file will lose metadata (like camera info, GPS). While in theory PNG could carry the same metadata, in practice converters rarely preserve it.
And a case for conversion: jpeg2png project, which doesn't merely convert the file, but also uses quite advanced post-processing to smooth out JPEG compression distortions. In that case you might salvage a low-quality blocky JPEG and get a smoother image instead.
The process you describe is:
JPEG Stream => JPEG DECODER => BITMAP
=> PNG ENCODER => PNG Stream
There is no change in step #2. Step #1 might have rounding errors inherent in JPEG.
I have an image with grayscale background and 2 thin lines (1 pixel wide) drawn in color. I'm trying to use various JPEG compression table (luminance and chrominance) to get the best possible result while staying under a certain file size.
The grayscale background compresses well and looks decent. The thin vertical and horizontal color lines get mutilated / smeared. The current JPEG algorithm uses 2x2 sub-sampling on the Cb and Cr channels and the chrominance compression table is fairly aggressive (high compression).
Is there any way to embed a "BMP" type data into JPEG image. Basically specify a color for specific pixels to be applied after the JPEG is de-compressed?
Any other ways clean up how thin color lines get encoded / decoded in JPEG without increasing the overall file size by a lot.
P.S. I'm testing all this stuff in Matlab.
I have an image captured via webcam of my cat (the subject might not be important). I've aquired it as a 31 kB JPG file. When I open it with an image editor, then save it (without alteration) as a PNG (max. compression) it stores as a 297 kB file.
Why is the PNG file 10x larger than the original JPG. As I understand it, opening a JPG is lossless, and saving a PNG is lossless. So, where does all the extra data come from? If the image comes entirely out of the small file, why does it then re-save to 10x the size on disc?
Please read this carefully. I'm not asking why the two formats produce different file sizes from an original image. I'm asking why opening an existing JPG then saving that exact same image as PNG is 10x bigger. I don't think that this is a duplicate question as far as I can ascertain.
Some tests I've done:-
I've looked at both JPG and PNG and they look identical.
I've zipped both files and got cat.jpg.zip as 31 kB, and cat.png.zip as 296 kB. I take this to mean that both files are fully compressed with virtually no latent redundancy.
I've tried this via the BMP format as well; cat.jpg (31 kB) -> cat.bmp (922 kB) -> cat.bmp.zip (404 kB).
Any ideas regarding the mysterious extra data..?
JPEG inherently produces better compression than PNG. However, JPEG trades off fidelity to the original image for better compression. PNG reproduces the original exactly.
If you go from JPEG to PNG, you are not going to see a changes.
If you go from PNG to JPEG, it is likely you ill see a lot of change.
JPEG uses a series of compression techniques. One of them, the DCT, transforms the image. This creates a subtle waviness in color. For example, if you start with a solid red block that is all one color, JPEG produces a lot of slight color variations.
PNG compression relies on finding repeated pixel patterns in scan lines. The subtle color variations introduced by JPEG can make PNG compression less effective.
The extra data you refer to is simply the difference in how the two format represent the same image.
If I take a JPEG image from a camera and convert to PNG, the result is usually about 10 times larger.
For a PNG Graphic image going to JPEG, I normally get files about 1/3 smaller.
JPG uses lossy compression, while PNG uses loseless compression. When you convert JPG to PNG, what actually happens is uncompressing from JPG and saving the results in PNG.
The "extra data" is actually due to different algorithms used.
As for why zipped files also have different size, that's because PNG has to save all pixels(including those JPG has lossy compressed) loselessy.
I have a matlab code and it generates a .png image of 1024*768 resolution. The images are about 450KB in size and I need to know how to optimise and compress these images using matlab.
Can't I play with the quality as in JPEG ?
I read the imwrite manual and don`t seem to find a good way to do this.
Is there any way to achieve it in matlab ?
By design PNG files are lossless - there is no 'quality' to be adjusted (it's probably why a mod changed your question title).
You can reduce the number of colors in the image (the color depth) which will in turn reduce filesize (PNG-8 instead of PNG-24, for example), but the whole point of PNG is it produces lossless images, so there is simple no quality value a la JPEG.
Taken from the manual :
A parameter of input in case it is JPEG:
'Quality' - A number between 0 and 100; higher numbers mean higher quality (less image degradation due to compression), but the resulting file size is larger.
imwrite(x,'c:\1.jpg','Quality',10)
edit: Sorry, I answered this one while the title was JPEG and not PNG.
PNG doesn't support any quality settings - it is a lossless format. The compression it applies is generally as good as possible.
JPEGs are smaller in size than PNGs. So, I thought that if I can make a specific region in a JPEG-file transparent, with some code, maybe I can save some bytes.
So does anyone know how to achieve this with for example PHP or JavaScript?
No. You can't do this. JPGs do not support alpha channels and have no capacity to designate certain colors as transparent either (GIF-style).
There's several issues with this, all of them have to do with that JPEG is a lossy compression format. The JPEG format is optimized for natural images and sharp edges will get blurred. If you intend that a specific pixel should have the value #d67fff there's no guarantee that after color conversion, FDCT, quantization, IDCT and color conversion, the pixel still will have that value. There's also a strong possibility that that pixel value will occur in areas that you don't want.
No. JPEG does not support transparency and is not likely to do so any time
soon. http://www.faqs.org/faqs/jpeg-faq/part1/section-12.html
You cannot do that, the client renders the image and doesn't know that you want it to treat that color as transparent (plus various compression methods on jpeg wouldn't work well with transparencies anyway).
I believe you can go with an 8-bit custom-pallet png, should save you a lot of space. Otherwise 24-bit PNG is your only high color option.
You can convert your image to SVG containing a color information as JPEG and an alpha channel as grayscale mask. Here is a tool I wrote to do it https://github.com/igrmk/transpeg