OT: Looking for JPEG/JFIF Gurus

U
Posted By
Uni
May 11, 2004
Views
747
Replies
4
Status
Closed
I posted this question before, addressed to Mr. Cox, believing he’d know the answer. Unfortunately and apparently he didn’t. Does anyone know what the purpose of the separate X & Y densities in the JPEG/JFIF standard? Some graphics applications offer access/modification to one value, while others offer access/modification to both.

Below, is my previous post on this subject…

Thanks!
——-

Hello Chris,

Maybe you can shed some light on this. Wayne Fulton and I are in disagreement about JPEG image density, in another Photoshop group. He claims, it doesn’t matter at all what it is, because it does not affect what is shown on the monitor. To some degree, I agree. However, I have one application (Vue Print) where it does matter. Making the X & Y densities different, changes the aspect ratio of the image on a monitor.

My questions: Does Photoshop read both X & Y density figures? Does it display them both in image information? If it only reads one value, is it the X or Y? Will Photoshop use only one of these values when printing?

Also, do you know why X & Y density figures were included in the JPEG/JFIF standard?

Thanks!

Here’s some snippets of the conversation….

It has an X & Y DPI of 21. This can be viewed with Irfanview’s Hex Viewer. To find the values, look for the "JFIF" zero terminated string, in the very beginning of the file header.
It’s the:
5th & 6th bytes = X density
7th & 8th bytes = Y density.
These are after the "JFIF" zero terminated string.
Uni

Wayne, if you take that image and modify its resolution within Irfanview’s image properties, maybe X=21 x Y=42, then save it, then open it with Vue Print, the image aspect ratio (monitor) will change. This MAY be a glitch with Ed H.’s viewer.

Uni

Regards,
Uni

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XT
xalinai_Two
May 12, 2004
On Tue, 11 May 2004 18:24:24 -0400, Uni
wrote:

I posted this question before, addressed to Mr. Cox, believing he’d know the answer. Unfortunately and apparently he didn’t. Does anyone know what the purpose of the separate X & Y densities in the JPEG/JFIF standard? Some graphics applications offer access/modification to one value, while others offer access/modification to both.

The JPG standard comes from a time where a lot of devices had non-square pixels and thus different resolutions in X and Y direction.

Neither NTSC nor PAL television uses square pixels (NTSC pixels are taller, PAL pixels are wider). Standard fax pixels are twice as high as they are wide (196xdpi 98ydpi), fine resolution uses square pixels(196dpi both).

If you want a circle to be round, you need to know the pixel aspect ratio – or the relation of x and y resolution when you draw it.

The need to respect this has been reduced when it was assumed that all computer devices had square pixels but is coming up again as some mobile device do use non-square pixels again.

Michael

Below, is my previous post on this subject…

Thanks!
——-

Hello Chris,

Maybe you can shed some light on this. Wayne Fulton and I are in disagreement about JPEG image density, in another Photoshop group. He claims, it doesn’t matter at all what it is, because it does not affect what is shown on the monitor. To some degree, I agree. However, I have one application (Vue Print) where it does matter. Making the X & Y densities different, changes the aspect ratio of the image on a monitor.
My questions: Does Photoshop read both X & Y density figures? Does it display them both in image information? If it only reads one value, is it the X or Y? Will Photoshop use only one of these values when printing?
Also, do you know why X & Y density figures were included in the JPEG/JFIF standard?

Thanks!

Here’s some snippets of the conversation….

It has an X & Y DPI of 21. This can be viewed with Irfanview’s Hex Viewer. To find the values, look for the "JFIF" zero terminated string, in the very beginning of the file header.
It’s the:
5th & 6th bytes = X density
7th & 8th bytes = Y density.
These are after the "JFIF" zero terminated string.
Uni

Wayne, if you take that image and modify its resolution within Irfanview’s image properties, maybe X=21 x Y=42, then save it, then open it with Vue Print, the image aspect ratio (monitor) will change. This MAY be a glitch with Ed H.’s viewer.

Uni

Regards,
Uni
U
Uni
May 13, 2004
Xalinai wrote:
On Tue, 11 May 2004 18:24:24 -0400, Uni
wrote:

I posted this question before, addressed to Mr. Cox, believing he’d know the answer. Unfortunately and apparently he didn’t. Does anyone know what the purpose of the separate X & Y densities in the JPEG/JFIF standard? Some graphics applications offer access/modification to one value, while others offer access/modification to both.

The JPG standard comes from a time where a lot of devices had non-square pixels and thus different resolutions in X and Y direction.
Neither NTSC nor PAL television uses square pixels (NTSC pixels are taller, PAL pixels are wider). Standard fax pixels are twice as high as they are wide (196xdpi 98ydpi), fine resolution uses square pixels(196dpi both).

Are you claiming a triad pel (Picture Element) is square?

Thanks!

Uni

If you want a circle to be round, you need to know the pixel aspect ratio – or the relation of x and y resolution when you draw it.
The need to respect this has been reduced when it was assumed that all computer devices had square pixels but is coming up again as some mobile device do use non-square pixels again.

Michael

Below, is my previous post on this subject…

Thanks!
——-

Hello Chris,

Maybe you can shed some light on this. Wayne Fulton and I are in disagreement about JPEG image density, in another Photoshop group. He claims, it doesn’t matter at all what it is, because it does not affect what is shown on the monitor. To some degree, I agree. However, I have one application (Vue Print) where it does matter. Making the X & Y densities different, changes the aspect ratio of the image on a monitor.
My questions: Does Photoshop read both X & Y density figures? Does it display them both in image information? If it only reads one value, is it the X or Y? Will Photoshop use only one of these values when printing?
Also, do you know why X & Y density figures were included in the JPEG/JFIF standard?

Thanks!

Here’s some snippets of the conversation….

It has an X & Y DPI of 21. This can be viewed with Irfanview’s Hex Viewer. To find the values, look for the "JFIF" zero terminated string, in the very beginning of the file header.
It’s the:
5th & 6th bytes = X density
7th & 8th bytes = Y density.
These are after the "JFIF" zero terminated string.
Uni

Wayne, if you take that image and modify its resolution within Irfanview’s image properties, maybe X=21 x Y=42, then save it, then open it with Vue Print, the image aspect ratio (monitor) will change. This MAY be a glitch with Ed H.’s viewer.

Uni

Regards,
Uni

XT
xalinai_Two
May 14, 2004
On Thu, 13 May 2004 16:54:42 -0400, Uni
wrote:

Xalinai wrote:
On Tue, 11 May 2004 18:24:24 -0400, Uni
wrote:

I posted this question before, addressed to Mr. Cox, believing he’d know the answer. Unfortunately and apparently he didn’t. Does anyone know what the purpose of the separate X & Y densities in the JPEG/JFIF standard? Some graphics applications offer access/modification to one value, while others offer access/modification to both.

The JPG standard comes from a time where a lot of devices had non-square pixels and thus different resolutions in X and Y direction.
Neither NTSC nor PAL television uses square pixels (NTSC pixels are taller, PAL pixels are wider). Standard fax pixels are twice as high as they are wide (196xdpi 98ydpi), fine resolution uses square pixels(196dpi both).

Are you claiming a triad pel (Picture Element) is square?

No. I’m not claiming anything. I am writing about assumptions about hardware that are used when programmers can not control the actual device used.

For computer monitors it is assumed that pixels are square by most applications and pixels definitely are square on LCD computer monitors.

For CRTs pixel aspects may vary depending on monitor geometry and resolution (especially 1280×1024 pixels on a 4:3 monitor will have pixels wider than high). Pixels should be square (use 1280×960!) but, depending on the mask type used, their actual shape may vary a lot.

Michael
U
Uni
May 15, 2004
Xalinai wrote:
On Thu, 13 May 2004 16:54:42 -0400, Uni
wrote:

Xalinai wrote:

On Tue, 11 May 2004 18:24:24 -0400, Uni
wrote:

I posted this question before, addressed to Mr. Cox, believing he’d know the answer. Unfortunately and apparently he didn’t. Does anyone know what the purpose of the separate X & Y densities in the JPEG/JFIF standard? Some graphics applications offer access/modification to one value, while others offer access/modification to both.

The JPG standard comes from a time where a lot of devices had non-square pixels and thus different resolutions in X and Y direction.
Neither NTSC nor PAL television uses square pixels (NTSC pixels are taller, PAL pixels are wider). Standard fax pixels are twice as high as they are wide (196xdpi 98ydpi), fine resolution uses square pixels(196dpi both).

Are you claiming a triad pel (Picture Element) is square?

No. I’m not claiming anything. I am writing about assumptions about hardware that are used when programmers can not control the actual device used.

Well, it just doesn’t make any sense.

However, thanks anyway.

Uni

For computer monitors it is assumed that pixels are square by most applications and pixels definitely are square on LCD computer monitors.

For CRTs pixel aspects may vary depending on monitor geometry and resolution (especially 1280×1024 pixels on a 4:3 monitor will have pixels wider than high). Pixels should be square (use 1280×960!) but, depending on the mask type used, their actual shape may vary a lot.
Michael

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