Appropriate Measuring

I was wondering if you have any information as to when it is (or is not) appropriate to measure on an image based on it's format and compression?  For example, can I know based on whether the image is a DICOM lossy image that it is inappropriate to measure on that image using the annotations ruler?  Is is true that just the fact that an image is lossy makes it inelligible for accurate measuring? 

What about a JPEG file with a quality of 2?  Is it true to that a JPG image with a FILE_JPEG_411 format and a quality of 2 (perfect) is lossy while a quality of zero is lossless?

Is there any way to know for sure if an image's aspect ratio has been altered in either direction which would make measuring inaccurate?

In my application, we have sonogram images which contain a scale from top to bottom usually along the right side of the image.  I can use that scale to "calibrate" to determine the exact DPI in the vertical direction which appears to provide accurate measurements (at least to the scale shown on the image).  However, if the image were to be stretched somehow in either direction, it would make sense that the measurements may not be accurate.

Do you know of any way to determine the appropriateness (or not) of measuring based on the images properties?

Hope you can help!

Barbara

Yes it is true that a JPEG 411 with a QFactor  of 2 is lossy while QFactor of 0 is lossless.

The quality factor and compression used shoudn't have any affect on your measurements, only on image quality.  The measurements of a ruler are based on the DPI of the image, which doesn't change when saving an image.  As long as your image dimensions and DPI don't change, your measurements will always be accurate.  However, if you repeatedly load and save an image with a lossy compression, you will lose some image quality and eventually your color information will be lost and/or blurred, but the ruler measurements themselves will still be accurate.

If you change the bitmap's dimensions, make sure that you change the DPI by the same ratio to ensure your measurements are accurate.  However, loading and saving alone will not stretch or shrink an image's dimensions.