by Chris Peterson » Sat Mar 30, 2013 3:11 pm
stephen63 wrote:Chris, speaking of that, perhaps you could answer this question. When I covert an image from TIFF format to JPEG(using photoshop), the color space is set to ICC Profile: sRGB IEC61966-2.1. Is this the working space most suitable for posting JPEGs on the web?
I always convert my web images to sRGB (if the sources aren't already in that space). Although the sRGB gamut is somewhat narrower than others, that's not usually a factor with images viewed in browsers, and sRGB is the assumed color space in most apps that aren't color managed. I figure using sRGB offers the best chance of the most people seeing something at least close to what I intend. Of course, it's also important to save the image with the color profile included, or results will be variable. This is a problem with many APOD images, which are frequently posted without color profiles. To be more precise, most of the submitters do provide images with color profiles, but the process used by the APOD editors to create the version that shows in the main page removes the color profile. When viewing APODs, it's important to click on the image to view the original if you want to see the correct color. (Would somebody chip in and buy those guys a copy of Photoshop? Last I heard they were using some 20 year old graphics package to produce the main page reductions/crops.)
As far as compression goes, I don't compress at all unless there are file size restrictions, like here. Is there a better way than using trial and error on the amount of compression until file size limitation is met?
This varies a bit between color and grayscale images, but as a rule, JPEG can compress about 10 times without producing visible artifacts with most photographic images (a setting of about 70 in Photoshop). If the intent is an image for display only (no later editing), saving at this level will almost always produce very good results- effectively indistinguishable from an uncompressed image. BTW, this applies to typical terrestrial images, which are full of high frequency information. Astronomical images often have little high frequency content. These can compress by a factor of 20 or more even when the highest JPEG quality is selected, with even highly compressed images being indistinguishable from uncompressed ones.
[quote="stephen63"]Chris, speaking of that, perhaps you could answer this question. When I covert an image from TIFF format to JPEG(using photoshop), the color space is set to ICC Profile: sRGB IEC61966-2.1. Is this the working space most suitable for posting JPEGs on the web?[/quote]
I always convert my web images to sRGB (if the sources aren't already in that space). Although the sRGB gamut is somewhat narrower than others, that's not usually a factor with images viewed in browsers, and sRGB is the assumed color space in most apps that aren't color managed. I figure using sRGB offers the best chance of the most people seeing something at least close to what I intend. Of course, it's also important to save the image with the color profile included, or results will be variable. This is a problem with many APOD images, which are frequently posted without color profiles. To be more precise, most of the submitters do provide images with color profiles, but the process used by the APOD editors to create the version that shows in the main page removes the color profile. When viewing APODs, it's important to click on the image to view the original if you want to see the correct color. (Would somebody chip in and buy those guys a copy of Photoshop? Last I heard they were using some 20 year old graphics package to produce the main page reductions/crops.)
[quote]As far as compression goes, I don't compress at all unless there are file size restrictions, like here. Is there a better way than using trial and error on the amount of compression until file size limitation is met?[/quote]
This varies a bit between color and grayscale images, but as a rule, JPEG can compress about 10 times without producing visible artifacts with most photographic images (a setting of about 70 in Photoshop). If the intent is an image for display only (no later editing), saving at this level will almost always produce very good results- effectively indistinguishable from an uncompressed image. BTW, this applies to typical terrestrial images, which are full of high frequency information. Astronomical images often have little high frequency content. These can compress by a factor of 20 or more even when the highest JPEG quality is selected, with even highly compressed images being indistinguishable from uncompressed ones.