How can I compress jpeg image with compression rate 4 bpp or less?

Question

I am trying to compress my .jpeg image in Photoshop. WHat is the best way to do this?

I am now calculating the bpp taking the image size in kb, calculating how many bits that is. Then I take the image size in pixel*pixel to get the amount of pixels in the image. After that I divide bits/pixels, to find how many bits per pixel the image has.

But How can I change this number? My guess is to change how many kb the image is, but how do i do this?

Thanks for any help!!

Answer 1

Yes, you can achieve higher compression ratio than 4 bits per pixel. Images with solid color can have rate as low as 0.13bpp.

In fact 4bpp is quite poor compression — it's same as uncompressed 16-color image or half of 256-color image, which even GIF can manage. JPEG can look decent at 1-2bpp.

Answer 2

in general, you cannot "compress" a jpeg image. all you can do is to reduce the image quality further in order to achieve a lower bpp value. jpeg streams are always compressed and they use a lossy compression method. it means that the original image will never ever be reconstructed from a jpeg image. the smaller the file the more information you have lost.

a specific "bpp value" is not, and should never be your target. especially with lossy compression. you should always look at your current image and decide whether it is still good enough or not.

if you still have the original image, try a lossless compression format, like zip-compressed or lzw-compressed tiff or compressed png. i'm sure PhotoShop can handle these formats as well. another softwares like IrfanView (https://www.irfanview.com/) or XnView MP (https://www.xnview.com/en/xnviewmp/) will convert your images too.

if you want manual (eg. full) control over your images, you should use command line utilities, like ImageMagick (https://imagemagick.org/) or NConvert (please find the XnView MP link above)

if you have only the jpeg images do not touch (edit & save) them. with every single save operation you lose another bunch of information. you should always work on file copies.

you should always keep your master image (the very picture you took with your phone or your camera).

of course, these rules of thumb will not answer your original question.