An image is broken down in picture elements (pixels) which means that when I check the attributes of an e.g. JPEG image stored on disk I can see e.g. 532 × 1051 pixels which means the width is 532 the height is 1051.
It also says in the attributes of the file that the color model is RGB which would mean 1 byte for each channel = 3 bytes to describe the color.
So I would assume that the actual size of the image is:
532x1051x3= 1.67MB
But what I see is that the actual file size reported is 45KB.
So does this mean that the 1.67MB is the size of the image in memory?
Also I see as part of the meta data: Image DPI: 96 pixels/inch.
What exactly is this information used for?
But what I see is that the actual file size reported is 45KB
That's because JPEG images are compressed in multiple ways to save up space, sometimes up to a factor of ten so this is not unusual. You can just search for "jpeg compression" and read how it is done in detail if you're interested.
Also I see as part of the meta data: Image DPI: 96 pixels/inch. What
exactly is this information used for?
Generally, as long as you are only interested in the image stored digitally you do not use it at all, because the image is not stored in inches. You could change the DPI and the image itself would not change, just the image header.
DPI only makes sense once you display/print the image. Then you could take it as a general recommendation on which resolution you should print this image. If you want to print a huge landscape with only 50 pixels per inch, it will not look good. If you however want to print a small logo with low DPI it might still look fine. So it really depends highly on the context of the image.
We're currently using apache fop to render a bunch of images in PDF format, but we've notice that the quality of just about any image is complete garbage and anything with text in it is incredibly unreadable.
I have looked through the documentation about source and target resolutions here.
And I thought that maybe I could force fit some good image quality in by assuming a default (source) resolution of 72 pixels per inch for all bitmaps/pngs/etc and desiring a target resolution of 300 pixels per inch. I figured this would theoretically cause FOP to cram way more image into a smaller space netting me an increase in quality with less of a need to smash my images into such a small space. Also I did the math and before where things were 72 72 it seemed like we were getting 5 * 72 = 375 pixel Nokia phone screen resolutions so I was like no wonder they are terrible quality.
Well, it seems like even with a target PPI of 300 instead of netting me a generous space of about 1500 pixels, I'm still getting the same crappy Nokia phone quality. I tried many combinations of source and target PPI 300/300 72/300 and 300/72 respectively, but nothing seems to cause any form or resampling or better image output. In the end I'm always left with this.
This is an actual screenshot of my PDF at almost full screen size. It may be worth noting that we are using the org.apache.commons.codec.binary.Base64 encoder to take images from memory and turn them into strings to embed into PDF. I don't know what kind of compression if any is being done through this encoder, but I hope to dump a string to the filesystem soon to take a look and make sure that isn't the issue.
In the meantime, does anyone know what PPI settings I might have messed up or what better options I may have to render clear cleaner images out to PDF with FOP?
I'm making a website with a single image as a background (with different backgrounds for subpages). So far I have established that the image should be about 1920x1080, possibly with 1.77:1 aspect ratio and a jpg for PCs. Now I want to reduce the image file size without losing quality.
1) First my problem. I have encountered the most bizarre thing in photoshop. When I upload an image 4272x2848 that weights 521 KB into photoshop and save it without changing anything, its size increases to... 1,52 MB ??? After I cut down the size to ~1920x1080 the size is still ~800 KB. Also the image before uploading has 96 DPI, after it is uploaded it changes to 72 DPI. (What sorcery is this?)
2) What is an acceptable image file size with that resolution?
3) Should I use save for web? This increases the size or reduces the quality from what I have experimented.
4) I found this image size reducer website: https://kraken.io/web-interface It reduces the size and I think the image quality does not change.
5) http://www.filedropper.com/pancakes - the image from question #1. (The image will probably be changed in the near future so this one is more of a case study).
Thanks!
JPEG being lossy, every time you load, then save, a separate JPEG algorithm is applied again. I believe the default for Photoshop is High quality, which an 8 on their dialog. So if you have an original JPEG that was originally saved as a low or medium quality (say a 4-6 on the Photoshop dialog), if you then open that in Photoshop, and go with the default "High/8" quality save, then the JPEG algorithm is applied on the perceptual image, meaning you saved a lower quality perceptual image at a higher quality algorithm's amount of data.
This is a major reason that I've moved away from JPEG. If JPEG is required I always try to start with either a RAW, BMP, TIFF, or PNG image, and then save off a JPEG version from that, then if I need to make any changes I go back to the full "original" [lossless] format, make the changes then save the JPEG again. I try to never edit an image that is already saved as JPEG, because you're always going to lose a small amount of quality (mostly the JPEG algorithm is good enough that the loss of quality isn't perceptual, but the file size can change none-the-less).
I am building a map system that requires a large image (native 13K pixels wide by 20K pixels tall) to be overlayed onto an area of the US covering about 20 kilometers or so. I have the file size of the image in jpg format down to 23 MB and it loads onto the map fairly quickly. I can zoom in and out and it looks great. It's even located exactly where I need it to be (geographically). However, that 25 MB file is causing Firefox to consume an additional 1GB of memory!!! I am using Memory Restart extension on Firefox and without the image overlay, the memory usage is about 360 MB to 400 MB, which seems to be about the norm for regular usage, browsing other websites etc. But when I add the image layer, the memory usage jumps to 1.4 GB. I'm at a complete loss to explain WHY that is and how to fix it. Any ideas would be greatly appreciated.
Andrew
The file only takes up 23 MB as a JPEG. However, the JPEG format is compressed, and any program (such as FireFox) that wants to actually render the image has to uncompress it and store every pixel in memory. You have 13k by 20k pixels, which makes 260M pixels. Figure at least 3 bytes of color info per pixel, that's 780 MB. It might be using 4 bytes, to have each pixel aligned at a word boundary, which would be 1040 MB.
As for how to fix it, well, I don't know if you can, except by reducing the image size. If the image contains only a small number of colors (for instance, a simple diagram drawn in a few primary colors), you might be able to save it in some format that uses indexed colors, and then FireFox might be able to render it using less memory per pixel. It all depends on the rendering code.
Depending on what you're doing, perhaps you could set things up so that the whole image is at lower resolution, then when the user zooms in they get a higher-resolution image that covers less area.
Edit: to clarify that last bit: right now you have the entire photograph at full resolution, which is simple but needs a lot of memory. An alternative would be to have the entire photograph at reduced resolution (maximum expected screen resolution), which would take less memory; then when the user zooms in, you have the image at full resolution, but not the entire image - just the part that's been zoomed in (which likewise needs less memory).
I can think of two approaches: break up the big image into "tiles" and load the ones you need (not sure how well that would work), or use something like ImageMagick to construct the smaller image on-the-fly. You'd probably want to use caching if you do it that way, and you might need to code up a little "please wait" message to show while it's being constructed, since it could take several seconds to process such a large image.
Consider an application handling uploading of potentially very large PNG files.
All uploaded files must be stored to disk for later retrieval. However, the PNG files can be up to 30 MB in size, but disk storage limitations gives a maximum per file size of 1 MB.
The problem is to take an input PNG of file size up to 30 MB and produce an output PNG of file size below 1 MB.
This operation will obviously be lossy - and reduction in image quality, colors, etc is not a problem. However, one thing that must not be changed is the image dimension. Hence, an input file of dimension 800x600 must produce an output file of dimension 800x600.
The above requirements outlined above are strict and cannot be changed.
Using ImageMagick (or some other open source tool) how would you go about reducing the file size of input PNG-files of size ~30 MB to a maximum of 1 MB per file, without changing image dimensions?
PNG is not a lossy image format, so you would likely need to convert the image into another format-- most likely JPEG. JPEG has a settable "quality" factor-- you could simply keep reducing the quality factor until you got an image that was small enough. All of this can be done without changing the image resolution.
Obviously, depending on the image, the loss of visual quality may be substantial. JPEG does best for "true life" images, such as pictures from cameras. It does not do as well for logos, screen shots, or other images with "sharp" transitions from light to dark. (PNG, on the other hand, has the opposite behavior-- it's best for logos, etc.)
However, at 800x600, it likely will be very easy to get a JPEG down under 1MB. (I would be very surprised to see a 30MB file at those smallish dimensions.) In fact, even uncompressed, the image would only be around 1.4MB:
800 pixels * 600 pixels * 3 Bytes / color = 1,440,000 Bytes = 1.4MB
Therefore, you only need a 1.4:1 compression ratio to get the image down to 1MB. Depending on the type of image, the PNG compression may very well provide that level of compression. If not, JPEG almost certainly could-- JPEG compression ratios on the order of 10:1 are not uncommon. Again, the quality / size of the output will depend on the type of image.
Finally, while I have not used ImageMagick in a little while, I'm almost certain there are options to re-compress an image using a specific quality factor. Read through the docs, and start experimenting!
EDIT: Looks like it should, indeed, be pretty easy with ImageMagick. From the docs:
$magick> convert input.png -quality 75 output.jpg
Just keep playing with the quality value until you get a suitable output.
Your example is troublesome because a 30MB image at 800x600 resolution is storing 500 bits per pixel. Clearly wildly unrealistic. Please give us real numbers.
Meanwhile, the "cheap and cheerful" approach I would try would be as follows: scale the image down by a factor of 6, then scale it back up by a factor of 6, then run it through PNG compression. If you get lucky, you'll reduce image size by a factor of 36. If you get unlucky the savings will be more like 6.
pngtopng big.png | pnmscale -reduce 6 | pnmscale 6 | pnmtopng > big.png
If that's not enough you can toss a ppmquant in the middle (on the small image) to reduce the number of colors. (The examples are netpbm/pbmplus, which I have always found easier to understand than ImageMagick.)
To know whether such a solution is reasonable, we have to know the true numbers of your problem.
Also, if you are really going to throw away the information permanently, you are almost certainly better off using JPEG compression, which is designed to lose information reasonably gracefully. Is there some reason JPEG is not appropriate for your application?
Since the size of an image file is directly related to the image dimensions and the number of colours, you seem to have only one choice: reduce the number of colours.
And ~30MB down to 1MB is a very large reduction.
It would be difficult to achieve this ratio with a conversion to monochrome.
It depends a lot on what you want at the end, I often like to reduce the number of colors while perserving the size. In many many cases the reduced colors does not matter. Here is an example of reducing the colors to 254.
convert -colors 254 in.png out.png
You can try the pngquant utility. It is very simple to install and to use. And it can compress your PNGs a lot without visible quality loss.
Once you install it try something like this:
pngquant yourfile.png
pngquant --quality=0-70 yourfile.png
For my demo image (generated by imagemagick) the first command reduces 350KB to 110KB, and the second one reduces it to 65KB.
Step 1: Decrease the image to 1/16 of its original size.
Step 2: Decrease the amount of colors.
Step 3: Increase the size of the image back to its original size.
I know you want to preserve the pixel size, but can you reduce the pixel size and adjust the DPI stored with the image so that the display size is preserved? It depends on what client you'll be using to view the images, but most should observe it. If you are using the images on the web, then you can just set the pixel size of the <img> tag.
It depends on they type of image, is it a real life picture or computer generated image,
for real life images png will do very little it might even not compress at all, use jpg for those images, it the image has a limited number of different colors (it can have a 24 bit image depth but the number of unique images will be low) png can compress quite nicely.
png is basicly an implementation of zip for images so if a lot of pixels are the same you can have a rather nice compression ratio, if you need lossless compression don't do resizing.
use optipng it reduce size without loss
http://optipng.sourceforge.net/
Try ImageOptim https://imageoptim.com/mac it is free and open source
If you want to modify the image size in ubuntu, you can try "gimp".
I have tried couple of image editing apps in ubuntu and this seemed to be the best among them.
Installation:
Open terminal
Type: sudo apt install gimp-plugin-registry
Give admin password. You'll need net connection for this.
Once installed, open the image with GIMP image editor. Then go to: File > Export as > Click on 'Export' button
You will get a small window, where check box on "Show preview in image window". Once you check this option, you will get to see the current size of the file along with Quality level.
Adjust the quality level to increase/decrease the file size.
Once adjusting is done, click on 'Export' button finally to save the file.
Right click on the image. Select open with paint. Click on resize. Click on pixel and change the horizontal to 250 or 200.
That's the only thing. It is the fastest way for those who are using Windows XP or Windows 7.