CODEWITHSUNDEEP
compressionjpeg
stevendesu
stevendesu

Reputation: 16791

JPEG: Dimensions versus Compression

Pretty simple but specific question here:

I'm not entirely familiar with the JPEG standard for compressing images. Does it create a better (that being, smaller file size at a similar quality) image when the X dimension (width) is very large and the Y dimension (height) is very small, vice versa, or when the two are nearly equal?

The practical use I have for this is CSS sprites. If a website were to consist of hundreds of CSS sprites, it would be ideal to minimize the size of the sprite file to assist users on slower internet and also to reduce server load. If the JPEG standard operates really well on a single horizontal line, but moving vertically requires a lot more complexity, it would make sense for an image of 100 16x16 CSS sprites to be 1600x16.

On the other hand if the JPEG standard has a lot of complexity working horizontally but moves from row to row easily, you could make a smaller file or have higher quality by making the image 16x1600.

If the best compression occurs when the image is a perfect square, you would want the final image to be 160x160

Upvotes: 3

Views: 1955

Answers (2)

jamesbtate
jamesbtate

Reputation: 1399

It looks like JPEG's compression ratio isn't affected by image dimensions. However, it looks like your dimensions should be multiples of 8 but in all your examples you had multiples of 16 so you should be fine there.

http://en.wikipedia.org/wiki/JPEG#JPEG_codec_example

If I remember correctly, PNG (being lossless) operates much better when the same color appears in a horizontal stretch rather than a vertical stretch. Why are you making your sprites JPEG? If they are of a limited color-set (which is likely if you have 16x16 sprites, animated or not), PNG might actually yield better filesizes with perfect image quality.

Upvotes: 4

Flipster
Flipster

Reputation: 4401

The MPEG/JPEG blocking mechanism would (very slightly) favor an image size that is an exact multiple of the compression block size in each dimension. However, beyond that, the format won't care if the blocks are vertical or horizontal.

So, the direct answer to your question would be "square is as good as anything", as long as your sprites divide easily into a JPEG compression block (just make sure they are 8, 16, 24 or 32 pixels wide and you'll be fine).

However, I would go a bit further and say that for "most" spites, you are going to have a smaller image size, and clearer resolution if you have the initial master image be GIF instead of JPG, even more so if you can use a reduced color palette. Consider why would you need JPG at all for "hundreds of sprites".

Upvotes: 4

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