Bulk image compression Mac and Windows (Before Upload)
If you have some high res images and wants to compress them before upload photoshop is one of the more powerful tool to do your compression. However to do it in bulk quickly I have found a better tool xnConvert. It does compression and resize.
Image compression vs file compression
Firstly do not get confuse with image compression and files compression they are different types of process altogether. Images compression (Lossy) uses algorithms to determine how best to reduce data/pixels to maintain the same viewing effect on a screen. Where else a file compression finds ways to reduce file size by looking at repetitive data stream and grouping them together and it works for any types of files. After a file is compressed they are commonly save as file types zip, tar, or gzip depending on their compression methods.
There are many criteria to reduce image file size. I typically work with these 3:
1 – Size
Size is tricky for images you want to keep at full width or full height and responsive, I typically just use the ratio of 1920×1200 for maximum prettiness and 1280×1024 for most cases and 800×600 for maximum file save (like a mobile intensive site). Anything else with a consistent size you can just use the chrome plugin MeasureIt to measure the size you need in pixels.
2 – DPI
We wont go into explaining Dots Per Inches (DPI) you can google and read up on that. Simply put , it is the amount of Pixels Per Inch (PPI) which is more commonly known as DPI from the printing era. They both mean the amount of image information per square inch.
All you need to remember here is that :
300dpi is good for hi-res large format printing like banners.
150 dpi is just good enough for a4 prints
75 dpi is sufficient for most web display (why not 72dpi)
3- File type
PNG (use for vector images or text or transparent bg)
Portable Network Graphics is a lossless file format created with the intent to replace the GIF format, due to the patent restrictions of GIF compression. The project was a success and we now have complete access to the format, which is patent-free, has great compression, and is widely supported by web browsers. PNG files are used primarily for transparent images, simple-color images, and images that have hard lines, like text and vector images.(not image, not gradient)
JPEG (use for photos or images with gradients)
Joint Photographic Experts Group created a file format, creatively named JPEG to handle complex-color photographic images. When saving a file as a JPEG, users have the choice of quality vs. compression. More compression results in a smaller file size, but you will lose quality. Obviously, less compression results in a larger file-size, but also a higher-quality image. The great thing about JPEG compression is that you can usually find a balance that both looks good and has a small file size. Unfortunately, JPEG files have no transparency. Additionally, the file format is lossy, meaning that it loses some of it’s data each time it is compressed.
GIF (use for animation)
Read and view here to see the compression differences
4 – Compression type
Depending on your file format they have their own compression, for more advance compression you can research the different type of compression algorithms available to different tools.
Batch Compressing images with xnConvert on your Mac and PC
1 – Choose files.
Cool thing here is that you don’t have to duplicate these files xnconvert will create a separate copy leaving your original copies intact.
2 – Choose compression criteria known as actions here in xnconvert.
A – Resize
B – set DPI
C – output>file format(jpeg) > settings