can I change the DPI on a picture

[Frank barry]

Good morning all, first of all I want to say thank you to everyone for their help

Question...I have lots of pictures but they have a low DPI (76) I can take them into Photoshop, save them from there with a higher DPI (300 / 600). I am wondering if this really changes them?
Cheers
Frank

[Mike Null]

Frank

I'm less than expert here but a low dpi photo is just that. There are times when you must use them and my practice is to re-sample them before editing. That is to increase the dpi using Corel Photopaint or CS6.

If you are trying to enlarge an image with this process you will be disappointed but it may be helpful in making the photo more engravable or printable. It is a poor substitute for a high res image.

[Chris Corwin]

Your best result for adding pixels to an image would be to make it blurrier. You can't add definition to a digital picture that didn't have it in the first place. I have rejected a lot of pictures over the past 4 years from people with crappy phone Picts.

I have set up a demonstration with a phone pict engraving and the same subject with a good dslr engraving to show the difference. Some people are ok with crap too. I just don't like making it.

[Sam Murdoch]

Your best result for adding pixels to an image would be to make it blurrier. You can't add definition to a digital picture that didn't have it in the first place.

That's the truth!

[Junior hall]

I was told the best way is to make sure that what your going to engrave vector is to make sure that the picture is at least 2mg or better 3mg is even better Also what others have said make it blurry I found making sure the picture is at least 2mg then vectoring it

[Scott Shepherd]

I was told the best way is to make sure that what your going to engrave vector is to make sure that the picture is at least 2mg or better 3mg is even better Also what others have said make it blurry I found making sure the picture is at least 2mg then vectoring it

That has nothing to do with any of it. You'd have to know the size you want to engrave it, along with the contrast before being able to give any numbers. A 2MB file size might be fine if it's a 3" x 3" photo, but if it's a 12" x 12", then it's not. Not to mention the colors in it. White and black take different amount of space, so a white graphic and black graphic will be two totally different sizes.

It's not that simple, by any means.

[Ross Moshinsky]

This topic is very confusing for most because people often use terms that shouldn't. DPI really doesn't exist in digital art. When you assign DPI to an image, it's actually just metadata which scales the image in software. The reality is, the image only has pixels. So when talking about digital artwork, you care about resolution (pixels) and quality.

Remember, a 9000x9000 100DPI image is exactly the same as a 9000x9000 300DPI image. The only difference is the 9000x9000 100DPI image will import into Corel/Illustrator/whatever will appear 3x the size. Scale is down 300% and the images will be exactly the same.

[Rich Harman]

White and black take different amount of space, so a white graphic and black graphic will be two totally different sizes.

I've never heard of black taking more space than white, or vice versa. I would need proof before accepting that as fact.

In an image each pixel is represented by a binary number that contains a certain number of bits (8, 16, or 24 are common). White will have a different value than black but the numbers will use the same quantity of bits - space taken up is the same.

[Clark Pace]

I've never heard of black taking more space than white, or vice versa. I would need proof before accepting that as fact.

In an image each pixel is represented by a binary number that contains a certain number of bits (8, 16, or 24 are common). White will have a different value than black but the numbers will use the same quantity of bits - space taken up is the same.

Easy to a test. Make a 2 color image(black and white). Save it, then invert the image, save it again, and see which one takes more memory.

[Kevin Nathanson]

Easy to a test. Make a 2 color image(black and white). Save it, then invert the image, save it again, and see which one takes more memory.

I'm not an engraver, but I was the director of technology for the world's largest printing company, and a group product manager at Adobe, where part of my responsibility was the imaging engine of Photoshop.

There is no difference in file size based on color value for an uncompressed file. The bit rate of the image (8bit (grayscale) 24bit (RGB color) or higher (HDR color and a few other fancy technical things like alpha channels, etc.) are constant, with each pixel being described by that number of bits. Files that are compressed with lossy compression (you know them as JPGs and PNGs) can vary based not on the color value but rather the sequencing of colors, as part of the compression algorithm is a process called run-length encoding, which takes into consideration the span of like or similar colors.

The latest version of Photoshop does a pretty amazing job of interpolation and sharpening in order to "res-up" files that are lacking in pixel content. This of course depends on the quality and sharpness of the source image.

K

[Scott Shepherd]

I've never heard of black taking more space than white, or vice versa. I would need proof before accepting that as fact.

Simple enough, take a digital camera, take photos with a lot of white in them and photos with a lot of black in them and look at the photo size. If it wasn't true, then every single photo you took would all be the same file size, which they aren't. The binary difference between representing white and black is quite different.

I agree, Photoshop files will be the same size, but photos aren't photoshop files.

[Scott Shepherd]

Kevin, lasers are only black or white. They don't see color.

[Rich Harman]

Simple enough, take a digital camera, take photos with a lot of white in them and photos with a lot of black in them and look at the photo size. If it wasn't true, then every single photo you took would all be the same file size, which they aren't. The binary difference between representing white and black is quite different.

I don't believe that the binary representation of black vs white occupies any difference in space. And photos taken with a camera are different sizes due to compression and the amount of detail in the photo.

Probably the reason that a dark photo takes more space is because it is not truly all black. It is many different shades of dark gray. Us an image editing program to adjust the levels and you will see lots of hidden detail emerge. Whilst a white photo, considered to be "blown out" can be nothing but white.

The pixels in a camera's image sensor are like little buckets that collect light. True black is when one of the buckets catches no photons at all - which nearly never happens. Even with the lens cap on the photo sites can collect some photons just from the heat of the electronics inside (noise). However, when the buckets fill up, such as when over exposed, well once it is full - it is full. Any additional photons won't make any difference - that is when you get pure white. So what you may see as black really is many, many shades of grey.

[Jerome Stanek]

Simple enough, take a digital camera, take photos with a lot of white in them and photos with a lot of black in them and look at the photo size. If it wasn't true, then every single photo you took would all be the same file size, which they aren't. The binary difference between representing white and black is quite different.

I agree, Photoshop files will be the same size, but photos aren't photoshop files.

That is not an exact way of finding anything out I can take a picture with a lot of white an very little black but have a lot of other colors in there. The only way would be the exact same black and white picture but with reversed image sort of like an M C Escher drawing

[Rich Harman]

Some more about colors and space they take up. Each pixel is composed of Red Green And Blue. In 24 bit color each of the colors will have a byte assigned to them.

That means that there are 256 levels of Red, 256 levels of Green and 256 levels of Blue in every pixel. Every pixel is described using three bytes, regardless of what color it is.

Pure black would have the lowest levels of each of Red Green and Blue which would be represented as zero's. 00000000 00000000 00000000. Pure white would be the highest value for each of the colors (255) - 11111111 11111111 11111111. Both Black and White use the same number of bits - they take the same amount of space on a hard drive.