Using GraphicsMagick to read in a PNG file and write it out under a different name is causing the file size to increase by more than double. This only happens to some PNG files and I can't figure out what is causing it.
Example problem PNG: http://www.google.co.in/logos/2011/verne-hp-3.png
I have tried everything from disabling the opacity channel to using different compression types to changing the interlacing type but nothing is helping me understand what is causing this problem.
If anyone has any suggestions on what might be causing this, your help would be very welcome.
Thank you.
The file is an indexed-color PNG with alpha stored in a tRNS chunk. When I rewrite this file to disk GM uses RGBA PNG, which causes the image size to increase.
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I'd like to batch convert .heic images (e.g. iPhone photograph) to .jpg files with imagemagick, with the goal of retaining as much of the quality from the original image as possible. To be clear, the resulting output size is not a concern.
I've been using
magick input.heic -quality 100% output.jpg
Is it possible to do better?
No, its not possible to do better per ImageMagick's website:
Set the quality to 100 to produce lossless HEIC images. Requires the libheif delegate library.
I interpret lossless as NOTHING is lost from original picture. However since you ARE converting to another file type maybe its possible you lose something, are you seeing any artifacts/issues?
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.
A few months ago I installed Image::Magick on my perl server, but now I have unexpected behaviour..
After hours of research, I'm stuck on some problem..
What I'm trying to do is convert an image from .png to .jpg.
There is no problem here.. the problem comes here. basically my converted file ".jpg" is always large than .png after conversion..
here a snippet of my code:
my $image = Image::Magick->new;
my $x = $image->Read($serverPathDir.'/'.$serverFileName); # an .png file
$x = $image->Set( Gravity => 'Center' );
$x = $image->Resize(geometry=>'435x210');
$x = $image->Set(Quality=>'80%');
$x = $image->Write($serverPathDir.'/'.'.jpg');
After some research and theory, I realized that such problem may exist in specific images.. when I test with other images, the result is more satisfactory but nothing like GIMP (png to jpg)... I don't know what compressor Image::Magick uses, but seems to be very different like others.. ie: GIMP
When converting to jpg and changing the dimension of the original, use a multiple of 8. So, change your resize to a geometry of '436x216' and see if that helps.
See here for more
The problem may have to do with PNG files that are already compressed.
When you convert to another format, you have to decompress the image. Then you compress it in another format, which may not be able to compress it as much as the original format.
Also PNG's with few colors can be compressed even more by using a color palette, containing only the colors actually used.
So it is actually quite likely to find a file where a JPEG version would be larger, if you have a large enough sample size.
You can read more about it on Wikipedia.
I would only convert to JPEG for files which are larger than some arbitrary size, ( Larger files are more likely to be a photograph. ) unless you absolutely need to use the JPEG format. ( For example a program which can't read PNG files )
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.
I have a bunch of diagrams created using a Java diagramming tool that I wrote - they are mostly black and white diagrams, with the blocks in aqua, and occasional other colours. They are currently being saved as JPG files, and I want to insert them into a book that I am preparing for Print On Demand.
The book is an OpenOffice ODT file, which will later be converted to a PDF.
Currently I use JPG files, but the print facility they use requires 300 DPI, so I modified my diagramming tool to set the xDensity and yDensity to 300, and resUnits to 1, using getAsTree(), and then expand the diagram by a factor of 3 (300/96). IMO the result looks pretty good!
Unfortunately, someone on another forum pointed out that line diagrams are "fuzzed" on JPG files, so suggested that I change over to PNG, or possibly BMP files, both of which ODT files allow to be inserted.
My problem is that BMPs don't seem to have a DPI, and PNGMetadata doesn't seem to support getAsTree(). Can someone point me in the right direction? Thanks.
I don't understand the getAsTree() part, but answering the question that appears in the title, setting dpi for PNG files, you could use the imagemagick convert tool:
convert -density 300 -units pixelsperinch infile.jpg outfile.png
PNG, BMP and dozens of other image formats don't compress your diagrams - compression is probably what your commentor was getting at. JPEGs are great for photos but suck at diagrams.
You might want to look into SVG and other vector formats. Or if your environment allows, exporting 0% compression JPEGs and converting them into another format for lossless reproduction at 300DPI.
Hope that helps!
I decided not to try to do this programmatically. Instead I create the original diagram in PNG, then convert to 300 DPI using Irfanview. Irfanview's batch capability lets me convert to 300 DPI, scale up to compensate, and set to grey scale, all in one operation - and on multiple files at a time. This seems to be the best solution - but thanks to everyone anyway!