Open Image Size and specify 50% ?
No, sorry, I mean half the original size – with same aspect ratio.
Resizing height & width to 50%, gives an image a quarter of the original size.
Half the original size is 50%.
If you want half the original AREA, then you need to do a small amount of math (dividing both sides by the square root of 2, or about 70.71%)
Well, I thought that as an image is 2D, the only definition of size would be area(²).
I did say my math was rusty!
ps: so resampling an image by ,say 200%, Quadruples the # of pixels?
Correct. The image size percentage is applied to the dimensions, since one is resizing to new dimensions. If you need to resize to a total number of pixels, you must take into account both dimensions, so you will need to multiply or divide both dimensions by the square root of the ratio desired.
ps: so resampling an image by ,say 200%, Quadruples the # of pixels?
If your file was ONE SINGLE pixel and you resampled it to 200% what would you expect to happen?
If your file was ONE SINGLE pixel and you resampled it to 200% what would you expect to happen?
Well, to give you the benefit of the doubt, and assume that that’s not a rhetorical/sarcastic question…
200% expressed as a number is 2; so 1 x 2 = 2
The point was, that having any percentage applied to both dimensions, then becomes an inaccurate value in total percentage terms.
then becomes an inaccurate value in total percentage terms.
Does a 4" x 4" become a 2" x 2" if you resample at 50%? Does a 640 x 480 become 320 x 240? Mine do. That’s 50% the size, isn’t it?
200% expressed as a number is 2; so 1 x 2 = 2
No, if you have a single pixel (1×1), and resample it 200%, you will wind up with 4 pixels, 2×2.
Another example.
If your image = 10×10=100 total pixels and you want 200 total pixels, then you would have to go to 14*14=196 pixels (you can’t get part pixels, so this is the closest you can get to 200 on a square). The 14 is the 70.71% figure mentioned by Chris, times two.
640 x 480 becomes 320 x 240, yes of course, but 640 x 480 is 4 times larger than 320 x 240, correct? What percentage would you call that?
What percentage would you call that?
In pixel area, it is 400%.
If you wanted half the area of the 640×480 = 307,200 you would use 452 x 339 = 153,228. I got those dimensions as 640*.7071= 452 and 480*.7071.
200% image size (based on dimensions). 400% image area and number of pixels. I would say, outside photoshop’s image size dialog, that 640×480 is four times the size of 320×240. But the image size dialog doesn’t ask for that kind of comparison. It asks what you want the H and W to be, and you can respond in pixels or percent. The foregoing conversion would be 200% H, 200% W.
Ok, back on track, initially I was wondering if there was a quick and easy way for Photoshop to ‘double’ or ‘halve’ the size(area) of an image – I take it this is all leading to ‘No’?
Well size is an ambiguous term then. In general, no one talks about area. They talk about dimensions.
So, now tell us – why would you want to double or halve the area of a given image?
So, now tell us – why would you want to double or halve the area of a given image?
I honestly can’t think of an answer – would you say that only doubling an image’s dimensions is valid, whereas by area is not?
Say (and this has happened) a client says that he wants to see some artwork at twice the size, and you show him it at 200%; then they point out that it’s 4 times the size?
then they point out that it’s 4 times the size
Then you say that they were not specific enough in their request, and do it the way they want, using the formula Chris Cox pointed out.
And no, there is no automatic way to get there, without pulling out the calculator.
then they point out that it’s 4 times the size?
Again, as Don points out, the term Size is ambiguous. What they are pointing out is that the area is 4 times the size, but the dimensions are twice the size. So who’s right?
IMO, if there is no utility in describing area with an image, then the definition of size defaults to dimensions (which is practical and professionally accepted).
It is not misleading then, to resample at 50% and say that the image is 50% of its original size. It may be 25% of the original area, or .125% of the original occupation of space on your hard drive, or any other non-relevant comparison that one would like to make, but still 50% of the standard unit of measure (dimensions).
Let’s ask this: What is the size of an image that is 150ppi? We cannot say unless we know its dimensions, right? Isn’t PPI a measure of area?
Ask the client what they mean by twice the size. When dealing with prints, for example, and they ask for twice the size of a 4×6, do they mean roughly 6×9 (twice the area) or 8×12 (twice the dimensions)? If the client is an engineer, he might be thinking area; if an artist, she might be thinking dimensions.
An artist would probably be thinking "Oh, about this big roughly".
I like the subtle gender switch!
