how convert pixel to mm for a mat file using Matlab?

Question

Neda 2019-5-20

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/463136-how-convert-pixel-to-mm-for-a-mat-file-using-matlab

评论： Walter Roberson 2019-5-21

I was wondering how can I convert pixel to the mm while my image is a mat file (uint 8). For example, I have a kernel size of 11 * 11 and another one a kernel size of 21 * 21, and I need to know what is equivalent of these sizes in mm?

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Walter Roberson 2019-5-20

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/463136-how-convert-pixel-to-mm-for-a-mat-file-using-matlab#answer_375889

Pixels have no inherent size. 11x11 matrix of pixels could be representing picometers or gigaparsecs. You need some kind of reference information.

7 个评论
显示 5更早的评论隐藏 5更早的评论

Walter Roberson 2019-5-21

perhaps you were given information such as that the image represents 217mm by 256 mm
perhaps you were given information such as that the image is 1/2 degree arc at 3.8 AU
in cases where you have an image file, sometimes there is header information such as EXIF information about what the pixel resolution is. Occasionally that information is not an outright lie, but unless you are using a scientific instrument such as a microscope, you should assume that the information probably has little relationship to reality
in cases where you have an image file, sometimes there is header information about aperature and lens length setting, or aperature and focal distance, that can permit you to calculate pixel resolution. Higher end cameras try hard to get the right focal distance because it is important for autofocus to get clear pictures, consumer cameras just get somewhere in the right range. However, most auto-focus is only accurate to about 3-ish meters; I see one of the Canon prosumer cameras indicating it has about an 8 meter autofocus range. There is also the problem that for any given lens settings, there is a distance beyond which any object effectively becomes at "infinity"
DICOM header information from a calibrated medical device tends to be pretty accurate
perhaps there is something in the image which is of exactly known size
perhaps there is something in the image which is of estimatable size. For example you can make out the pixel distance between the centers of the pupils of a person being photographed, and you are able to estimate their age and gender, you can use reference statistical tables to estimate the real-world distance represented

Walter Roberson 2019-5-21

The information there is not enough to convert pixels to mm.

It looks to me as if you have a reconstructed ultrasound image with minor variations in coordinates, perhaps due to round-off in the reconstruction. There does appear to be a small scale difference between x and y.

Unfortunately, ultrasound is a technology used on a variety of scales, including for small scale welding, and large scale welding, and fetal monitoring. Just knowing the technology involved does not give us any scale information.

You could guess that the coordinates are in mm and see whether it gives you reasonable conclusions. Is it plausible that you are looking at something that is about 6.6 inches by 5.6 inches?

Walter Roberson 2019-5-21

You could multiply number of pixels by mean(diff(info.x)) or mean(diff(info.y)) to get "units" for which there is still a missing scaling factor to convert to millimeters, with it being plausible that the coordinates reflect millimeters directly.

However if the small differences in coordinates is important, you could get a more accurate area, except for the pixels on the right or bottom edge: you can deduce pixel widths by looking at differences in adjacent coordinates, but you do not have the coordinates for what is beyond the right or bottom edge in order to deduce the widths of those pixels.

请先登录，再进行评论。

how convert pixel to mm for a mat file using Matlab?

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

采纳的回答

7 个评论
显示 5更早的评论隐藏 5更早的评论

更多回答（0 个）

另请参阅

类别

标签

Community Treasure Hunt

how convert pixel to mm for a mat file using Matlab?

0 个评论 显示 -2更早的评论隐藏 -2更早的评论

采纳的回答

7 个评论 显示 5更早的评论隐藏 5更早的评论

更多回答（0 个）

另请参阅

类别

标签

Community Treasure Hunt

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

7 个评论
显示 5更早的评论隐藏 5更早的评论