Optimize image to filesize with GDLIB?

Toby

The question is: can it be done? When users upload files they should be optimized to a certain filesize...

bobthedog

I assume we are only talking about images here....

it seems that optimizing to an exact file size (as opposed to fixed image dimensions) isn't that easy to do (this is assuming you are only going to use PHP's built in functions and can't or won't be able to find a seperate 3rd party library to do what you want). Firstly, it is going to depend on the physical dimensions of the image - try getting a 1000px x 1000px image with a large set of colours down to anything under 10k - it aint gonna be done. Additionally - the more you look at reducing the files size, the more image degredation you will get, and in some cases this will probably negate the value of uploading an image to the site.

Remember - making the most of image optimisation means a manual process to get the balance of offsetting quality and filesize right.

Of course - if this is still something you want to do then take a look at the [man]imagejpeg/man function. The 3rd parameter in this is the "quality" of the final jpeg produced. Obviously the lower this is (allows values from 0-100 with 100 being the best), the smaller the final file size will be, and the more the image will degrade.

An alternative would be to restrict the file size that a user can upload to the site. When a file gets uploaded, check the size of it, and only accept it if it fits your preset size limit, you could also do a dimension check if you were worried about large dimension images being uploaded.

Anyhow - hope this points you in the right direction!

Toby

hi bob, thank you for taking the time..

Yeah, I should have said image, not file 😉
The problem with filesizes is that I am allowing users to upload a zip/rar-file which will then be extracted, all non-images removed and the rest is then put in the right folders on the filesystem, a thumb is created and the data is stored in the DB.
Now I don't want to check the images filesizes after uploading them one by one. I would rather manipulate them to reach a certain file size.
The problem with setting the jpeg-quality to a certain percentage is that if I set it higher than it actually is, I will get an increased filesize, right? Basically what I need to know is if there is a way to roughly predict (average) filesizes given a certain width and height and image quality.

Also: can you retrieve the present jpeg quality of an image to avoid making it bigger than it was?

bobthedog

As far as I know, there is no way of extracting a JPEGs quality from the file (this is all assuming you use JPEG compression for your images), sicne this doesn't seem to be one of the things stored in any of the information sectors.

The problem with setting the jpeg-quality to a certain percentage is that if I set it higher than it actually is, I will get an increased filesize, right

Indeed you will - this is going to be a risk you have to run.

a way to roughly predict (average) filesizes given a certain width and height and image quality.

Again - this is probably something that is fairly difficult to do, since file size depends on the dimensions fo the image, and the number of colours present in the image, as well as the allowed quality setting when making the JPEG.

The only way I think you could get what you require, is to perform a set of functions on each image, producing different files for a range of predefined quality settings, and determining which one you are willing to accept. However, this is going to be a lengthy process to run and will eat a lot of processing time - and I don't think your end results will be as good as you expect (again down to degredation of the image as the quality is reduced).

Toby

thx bob. thought that you could get jpeg-quality, but if you can't there is probably nothing i can do...

i'll have to check out imagemagick... maybe, hopefully ... 🙂

bobthedog

You are getting JPEG quality - but what is JPEG quality. JPEG is just a compression routine run on images to make them smaller, otherwise we would all be dealing with massive Bitmap files.

Therefore, there are different values of quality you can run through the routine in order to make smaller files - with the penalty of lowering the quality of the final image.

Unfortunately - this tends to be something which you can only reliably do by eye, since you will be able to tell what quality works for a particular image. Because of differing coloursets, image dimensions and required final results, there is no such thing as a generic quality which will work for all images (otherwise what would be the point in having a quality variable!)