Aliasing problem on JPEG (I think)

deisenberg wrote on 10/7/2003, 3:43 PM

Anyone,

I'm a new user of Screenblast (the latest version). I'm trying to do something very specific and am running into trouble.

I'm creating a slideshow of a family event. One of the photos in the slideshow is a high-resolution JPEG of the invitation to the event. This invitation was designed in Quark; the designer exported the design as an EPS file, imported that EPS into Photoshop, then exported the image as a high-resolution JPEG for me to incorporate into my slideshow. (I only mention this to clarify that the JPEG isn't a digital photo, if that makes any difference.)

My problem: I'm using the Pan/Crop tool to zoom in on the invitation, and the resulting DVD looks really bad on that particular picture. I don't think that the problem is with the MPEG compression, because even the rendered AVI in Screenblast doesn't look very good. It's hard to describe what I'm seeing; the video is very choppy, because it's as if the TV doesn't have enough horizontal scan lines to allow a smooth zoom-in effect. It looks as though the text of the invitation floats a little bit up and down as it's trying to stuff itself into the available scan lines of the TV (or monitor). I should mention that the other images in the slideshow (which are all high-resolution photos) look pretty good in the rendered DVD. It's only this invitation that looks bad.

I hope that what I'm describing makes sense (I'm a total newbie at this). Is there anything I might do to make the zoomed-in (or out) image smoother? Any settings I can play with? The designer can also recreate the JPEG for me in any manner I request, if that's will help.

Any advice would be greatly appreciated. . . thanks!

David

Comments

pete_h wrote on 10/7/2003, 5:01 PM

Sounds as if the resolution of your invitation is the problem.

Does it look ok without the 'zoom' effect?

deisenberg wrote on 10/7/2003, 8:52 PM

Pete,

The image does look okay to me without the "zoom" effect. The resolution of the JPEG is 1200 x 1725 pixels, with a 24-bit color depth. Shouldn't that be a high enough resolution?

David

Chienworks wrote on 10/7/2003, 9:23 PM

It depends on how far in you're zooming. The standard video frame size is 720x480. If you zoom in to an area of your JPEG file smaller than that, then you'll see the pixels being magnified.

deisenberg wrote on 10/8/2003, 7:09 AM

I e-mailed the designer of the invitation (who prepared the JPEGs) with all of these comments, and here was her response (I'm posting it because I'm a little out of my depth here):

"Hmm. This is a little confusing. Photoshop labels resolution differently than cameras or programs relating to them--I believe the numbers you're referring to as resolution are actually what I'm calling physical size. I don't have copies of what I sent you, but the number you're giving me, 1388 x 2100, was the size of the image, approx. 4 2/3" x 7". I think that all programs associated with cameras/movies automatically assume that images are 72 pixels/inch--the number that designers refer to as "resolution"--because that's what screens display, and so they automatically convert everything into this resolution. Some programs, however, will interpolate the images instead--so an image saved at higher resolution will become correspondingly physically larger when converted to 72pixels/inch if it starts out at, say 300/ppi (which is, to my recollection, the resolution I sent you, as it's print resolution and these started out as documents for print). But other programs just convert the image to 72/ppi and keep the physical size the same as it was. I think that's what your program did, although I was betting it would do the other thing. At 300 pixels/inch, , if interpolated, the resolution of the images would remain very high (i.e., at 72/ppi, the physical size of this image would actually INCREASE by about 300%). Make sense? So, yes, it looks like I have to increase the physical size of the images (resolution not a concern) in order to get you what you need. The next question for you to ask your
user forum is: how much larger should I save them? double, triple, etc?"

Does this make sense? Do I need what the designer is calling "larger" images? I guess what I'm asking is: for what should I ask her?

Thanks so much!

David

JohnnyRoy wrote on 10/8/2003, 7:43 AM

My guess is that you are zooming in quite a bit to show the details of the invention. If you don’t want any pixilation, then you need to make sure that the tightest zoom is near 720x480 in size. (it can be a bit smaller but not too much smaller)

So let’s say this is a picture of a car and you want to zoom in to show the fine details of the tire. If the tire takes up 1/10th of the picture then the resolution of the image needs to be 10x 720x480 or 7200x4800 so that when you zoom in 10x to see the tire you still have enough resolution. Does this make sense?

Alternately, you could use several images. One of the whole invention, and one of each of the areas you want to zoom into already at the 720x480 resolution. Then you can begin a zoom on the big picture and just cross fade to the higher resolution picture. The audience will still get the effect of where on the invention you are zooming to and also have a nice high-resolution image of the zoomed area.

If you want to get fancy, you could keep the main image still and use an effect to zoom up to the detailed image. So instead of zooming in, have a circle appear on the screen where the details are, then have that circle expand out revealing the high-resolution image of that area. Sort of like a magnifying glass effect. Use the Iris transition with a colored border.

~jr

Steve Grisetti wrote on 10/8/2003, 8:29 AM

As usual, JohnnyRoy is right on the nose!

At your present photo's resolution, you will able to zoom in about halfway before it will pixelate. JohnnyRoy has offered excellent work-arounds for going in deeper.

Of lesser issue is how JPEG's compress images by averaging the color values of the pixels. At high compression, particularly with photos, this isn't much of an issue. But, if your designer did some heavy compression to save file size, it can add ugly "artifacts" to the image, which will be more obvious when you zoom in. If you image is something other than a photo (say, a solid color object on a solid color background), these artifacts can make for a raggy edge where the colors break. This is the reason bitmaps or TIFFs are recommended if you plan to chromakey out a section of your image rather than a JPEG.

Yeah, I know. More than you needed to know. But maybe some day it will come in handy.

deisenberg wrote on 10/9/2003, 11:59 AM

Thanks so much for the information, everyone. I believe I understand the issue; I've asked the designer for a higher-resolution image, and I'll try that. (If it still looks bad, I'll be back on the forum. . . <g>)

David