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09/20/2005 10:44:01 AM · #1 |
When I take my image 640 x 520 and try to save as jpeg optimized to file size at 150K the quality drops all the way to 50% and looks terrible. Most of the time I don't have that problem. Any idea why this would occur. The image does have a lot of detail and very little repetition so I suppose the compression algorithm isn't very efficient.
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09/20/2005 10:47:10 AM · #2 |
Try lowering it to 620 or 610. It really isn't that noticible when posted.
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09/20/2005 10:52:47 AM · #3 |
the more detail there is the more data the image contains - therefore the file size will be larger. i have never had to compress more than 75% for a 640px image.
Message edited by author 2005-09-20 12:36:19.
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09/20/2005 10:53:41 AM · #4 |
at 640px, if you're getting 50% when optimized to 150k, there must be either a ton of detail or a lot of grain in the shot. Enebn at 50% quality the compression is usually not very noticeable. Normally a larger image with slightly lower quality setting looks better than a reduced-size image at higher quality. I've actually never encountered an image that, at 640x640, was impossible to acceptably compress to 150k.
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09/20/2005 10:56:55 AM · #5 |
| reducing it to 620 allows me to at least get to 61% |
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09/20/2005 10:59:28 AM · #6 |
I have had to reduce quite a few of mine (particularly the more square ones) to 610 to get the quality above 70%. Not one person have ever commented that the image was too small.
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09/20/2005 11:00:17 AM · #7 |
Here is a test I did on a high-detail image at various image dimensions and qualities. The target file sizes are stated, and the quality is encoded in the file name, e.g. "q43" means quality was 43.
the image used was intended to test both degradation of high-frequency detail and artifacting in very smooth areas (in this case sky) and to test for the appearance of "files" around areas of detail. At 640x640, even a quality level of 43% resulted in very little visible artifacting.
Edit:
Notice how a larger image size produces a perceptibly higher quality image, even at very low quality settings. While I don't recommend submitting to a challenge at quality = 19, the test results indicate that the JPG compression algorithm is more robust at lower quality settings than we usually give it credit for.
Message edited by author 2005-09-20 11:04:33.
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09/20/2005 11:11:32 AM · #8 |
| This might be obvious, but make sure you are at 72 dpi. If you are trying to save at a higher resolution you will have a larger file and be wasting space as far as the 'net is concerned. |
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09/20/2005 11:14:17 AM · #9 |
Originally posted by DrAchoo:
This might be obvious, but make sure you are at 72 dpi. |
Umm... no. Resolution is irrelevant. It is just a number assigned to an image but has no bearing on how an image is displayed in a web browser. The only thing that matters is the dimension in pixels. 640 x 400 at 72 dpi is identical (except for the "resolution tag") as 640 x 400 at 300 dpi. |
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09/20/2005 11:16:20 AM · #10 |
| D'oh! yes, that makes perfect sense now... |
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09/20/2005 11:48:04 AM · #11 |
Originally posted by EddyG:
Originally posted by DrAchoo:
This might be obvious, but make sure you are at 72 dpi. |
Umm... no. Resolution is irrelevant. It is just a number assigned to an image but has no bearing on how an image is displayed in a web browser. The only thing that matters is the dimension in pixels. 640 x 400 at 72 dpi is identical (except for the "resolution tag") as 640 x 400 at 300 dpi. |
I beg to differ. I agree with DrAchoo. Resolution is indeed important. Typically web browsers/computer displays at 72 dpi hence the suggestion. It doesn't matter if you set your monitor at 1280x1024, it's still the same.
Try saving the file using "Save for Web". That knocks down the file size quite rapidly because it optimizes the image for web quality images. My images on my protfolio and the ones in challenges are all done this way and my file sizes are way below 150k. |
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09/20/2005 11:58:17 AM · #12 |
Originally posted by rikki11:
I beg to differ. |
Feel free to differ. But I stand by my statement. I can set my JPEG to 10 dpi, 100 dpi or 1000 dpi and it will not make any difference as to how it is displayed on-screen, or changes the size of the file, or change the quality, or anything else. The DPI value on a JPEG is just an arbitrary number. Some Canon cameras create JPEGs with a DPI value of "180". Does that mean the camera is "only 180"?! Of course not. It is just an arbitrary value assigned to the pixel data which can be changed at will with no ill effect to on-screen display. What it will effect is how big the image is when you physically print it out.
Message edited by author 2005-09-20 12:06:48. |
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09/20/2005 12:04:44 PM · #13 |
Originally posted by rikki11:
Originally posted by EddyG:
Originally posted by DrAchoo:
This might be obvious, but make sure you are at 72 dpi. |
Umm... no. Resolution is irrelevant. It is just a number assigned to an image but has no bearing on how an image is displayed in a web browser. The only thing that matters is the dimension in pixels. 640 x 400 at 72 dpi is identical (except for the "resolution tag") as 640 x 400 at 300 dpi. |
I beg to differ. I agree with DrAchoo. Resolution is indeed important. Typically web browsers/computer displays at 72 dpi hence the suggestion. It doesn't matter if you set your monitor at 1280x1024, it's still the same. |
The first part of this statement is not true. Monitors display at 72 PIXELS per inch, for one thing (some actually go to 100 or 125 now btw), and it's just a description of the mapping function, for another.
What do I mean by "mapping function"? The monitor "maps" an image pixel-by-pixel. One pixel on the image = one pixel on the monitor. If your image is 640 pixels wide and your monitor is set to a 640-pixel horizontal resolution, the image fills the screen. Set the monitor to 1280 and the image uses half the horizontal dimension of the screen. Set monitor to 2560 and it uses a quarter of the screen horizontally.
Period.
For display, the only thing that matters is the actual pixels transmitted, not the dpi or ppi coding appended to them.
Robt.
Message edited by author 2005-09-20 12:06:30.
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09/20/2005 12:08:50 PM · #14 |
I'm gonna agree with bear and eddy here. the 72ppi is not relevant to the quality of the image displayed on a monitor vs any other ppi you have the file saved at because that's not displayed as the file as saved, but only as the monitor displays pixels.
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09/20/2005 12:12:51 PM · #15 |
Just to reiterate the point, it's beyond "agreeing" or "disagreeing"; it's an absolute fact. Your screen maps whatever is transmitted to it pixel-by-pixel. Ten pixels in the image = 10 pixels on the screen. The only variable is how many pixels your SCREEN displays per inch, and you set that at the monitor level, not at the photoshop level.
DPI is only relevant when printing.
Robt.
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09/20/2005 12:31:37 PM · #16 |
Originally posted by bear_music:
Just to reiterate the point, it's beyond "agreeing" or "disagreeing"; it's an absolute fact. |
If only life were that easy. ;)
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09/20/2005 12:39:30 PM · #17 |
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09/20/2005 12:57:26 PM · #18 |
| heh, even I agree with them now. I'm not sure what I was thinking... |
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