Kodak Photo CD's Lost Highlights Problem By: Ted Felix

Introduction

When loading Kodak Photo CD images into an image editing application, you will quickly discover that not all imaging applications are created equal when it comes to Photo CD. I've seen many complaints about "washed out" images when loaded into Photoshop, and bad attempts at explanation by the labs that produce these scans. On this page you will learn the real story.

Washed Out

I took a roll of film to the Kodak lab for scanning, and got back several scans that looked horribly washed out when loaded into Photoshop:

Washed Out.

This had happened in the past, but not quite as badly as this one particular time. At first I wasn't sure whether it was me, or Kodak. I knew I was using my on-camera flash in automatic mode, and that it was somewhat unreliable. But, on close examination of the negative, there was plenty of detail in these blown out areas. So I decided it had to be Kodak's fault. I was really fed up, and swore I would never use Photo CD again. After I cooled down a bit I thought I would look a little more closely at the problem and see if there was some solution.

If you ask some folks about this problem, you'll hear all sorts of explanations. Some blame it on the Gamma of the Photo CD images vs. the typical PC monitor Gamma. Others claim that if you load the images into Lab mode (set your destination profile to CIELAB) this problem goes away. Still others recommend you use the Kodak Photo CD Acquire Module (3.0.2 is the latest I tested). Some suggest upgrading the Kodak Photo CD File Format Plug-In to Kodak's latest version 3.0.7. I tried all of those things, and kept getting the exact same results: Lost Highlights.

ThumbsPlus

Then I tried loading this same image with ThumbsPlus and noticed a striking difference:

Loaded with Photoshop

Loaded with ThumbsPlus

The ThumbsPlus version isn't great, but it is definitely better. You don't need a histogram to see that Photoshop is losing a significant amount of detail compared to ThumbsPlus.

Fortunately I was familiar with Kodak's Photo CD reference image which can be found in their PCD-102 white paper. So I decided to use it as a standard to test various applications to see how much highlight information was being lost. You can see the analysis I did in the "Dangerous Curves" section below.

The Highlight Compression LUT

At this time I was also carefully reading all of the technical information about Photo CD that I possibly could. Being a Mathematician and Software Engineer, I had no trouble deciphering Kodak's technical documents. Two things caught my eye as I was reading. The first was Kodak's PCD-042 white paper and specifically table 3. This table details a curve to be used for compressing highlight information in a Photo CD image. That seemed like a pretty good lead.

Later I picked up a copy of Giorgianni and Madden's book "Digital Color Management" which on pages 291-292 also describes this same highlight compression technique.

In a nutshell, Kodak recommends that applications use a Lookup Table (LUT) after converting the PhotoYCC data from the Photo CD into RGB. Apparently the conversion yields numbers that range from 0 to 346 instead of the usual 0 to 255. So this Highlight Compressing LUT should squeeze the 0 to 346 numbers back into the 0 to 255 range. By the end of all my experimentation, I discovered that this was indeed where the washed out/lost highlights problem lies.

Picture Window

It was at about this time that I was complaining on USENET's rec.photo.digital newsgroup, and someone mentioned a software package called "Picture Window" that might help. Eager to find a solution I installed Picture Window and ran my usual set of tests with the reference image. I was absolutely amazed. As I knew might be possible, Picture Window loaded the image with no highlight loss at all.

ThumbsPlus: Best so far

Picture Window: Wow!

Having noted Picture Window's victory in solving the Lost Highlights Problem, I went to work to see if I could solve it myself by modifying Kodak's software. I specifically targeted Kodak's PCDLIB32.DLL which is used by many applications such as ThumbsPlus and Paint Shop Pro to load Photo CD images. After an hour or so, I had found the Lookup Table (LUT) that was responsible for the data loss, and changed it to preserve the highlights like Picture Window. With this patched Photo CD library, I can now use ThumbsPlus to load Photo CD images with no highlight loss at all.

Picture Window

Patched Photo CD Library

Note that Picture Window adjusted the black point automatically, and upped the saturation quite a bit. My patch gives a more realistic result similar to the unpatched library. All the highlight data is present in both images. If Picture Window does its math right, you should be able to get more shadow detail using Picture Window than you can with my patch.

Photo CD Player

I've always known Kodak made a Photo CD player at one time. Out of curiosity, I searched ebay and found one. $50 later I became the owner of it. The moment it arrived, it was a rush to find out how the player would treat my washed out image. The result was unexpected. The Photo CD player worked like Picture Window. So whoever implemented the Photo CD player must have realized that Kodak's highlight compressing LUT was a bad idea, and removed it. Good for them! I'm glad somebody agrees with me and Picture Window.

Of course, I have no way of really analyzing the Photo CD player's algorithm since I can't get the reference image into it, and even if I could, how would I measure the output?

Who's right?

Now we have three different ways to load a Photo CD image. Each of them differs in the severity of their Highlight Compression LUT. Picture Window has a linear Highlight Compression LUT and will not compress away any highlight detail from an image. ThumbsPlus and Paint Shop Pro use a moderate Highlight Compression LUT that does lose some highlight information. And lastly, Photoshop weighs in with the most extreme Highlight Compression LUT that destroys a significant amount of the highlight data quite effectively.

While digging deeper into the Photo CD system, I discovered exactly where the responsibilities lie for these behaviors. Photoshop uses Kodak's Photo CD File Format Plug-In (hereafter referred to as "the Plug-In") to load Photo CD images. This is where the extreme Highlight Compression LUT lives that causes the horrible results with Photoshop. ThumbsPlus and Paint Shop Pro both use Kodak's PCDLIB32.DLL toolkit to load Photo CD images. PCDLIB32.DLL has the moderate Highlight Compression LUT. Picture Window does its own thing, and can be considered in a separate class that it shares with the Photo CD player.

Now for a list of questions that only Kodak can answer:

• Why is the Plug-In's Highlight Compression LUT so extreme?
• Why is the Plug-In's Highlight Compression LUT different from PCDLIB32.DLL's?
• Why doesn't the Photo CD player use a Highlight Compression LUT?
• Why do I get so much email from people thanking me for my version of PCDLIB32.DLL with a LUT like Picture Window, and for pointing them to Picture Window?

What to do?

If you've been having trouble with blown highlights, I suggest you go through my list of Software that Really Supports Kodak Photo CD. There you'll find quick reviews of a number of software applications that let you get all the highlight information out of a Photo CD image. Most of them have free trials you can download to see which one suits your needs the best.

Dangerous Curves

If you're into curves, here's the analysis I did of the luminance values after loading the reference image with various software packages.

Luminance Curves when loading the reference image with different software packages.

Note that Picture Window is linear, but with a little bit of loss in the shadows. This is probably Picture Window's Auto-Levels algorithm which adjusts the black point to some small percentage of the shadow information. You can see in the example images above that Picture Window does produce a somewhat dark image. The advantage is that for the remaining image data, you should get more detail. Still it would be nice to be able to set the black point percentage in Picture Window's Auto-Levels algorithm.

ThumbsPlus (and all other software that uses PCDLIB32.DLL) shows highlight compression, and shadow/midtone expansion. The compression kicks in around 192. Paint Shop Pro falls into this category.

Photoshop (PS5 sRGB) is similar to ThumbsPlus, but much worse. The highlight compression starts to kick in at around 160. This causes a severe loss of highlight information. This result applies to Photoshop 5.5 and all prior versions. It will probably apply to all future versions unless by some miracle Kodak changes the Photo CD File Format Plug-In. You might as well just wait 'til Hell freezes over.

The Lost Highlights Problem is most noticeable with color and b/w negative scans. With transparency scans, it is not as bad, but it is still a serious problem (especially in Photoshop). Check out my Photo CD and Transparency Film page for more info.

Technical Notes

I tried to avoid any kind of processing on the images shown so you can see the effects of Kodak's Photo CD software when loading images. I stuck to the sRGB color space, performed no density adjustments, resized the images and saved them as JPEG, quality level 6. What you see here is very close to exactly what you would get when loading Photo CD images as described.

If you'd like to run the tests yourself to verify my results, or you'd like to test your favorite imaging app, go to my Photo CD Experiment page for all the details. There you can download my test images and follow my steps to reproduce the problems.

Macintosh

Please note that I am a PC user. I'm not sure how Photo CD behaves on the Mac. Keith Richmond tried running my experiments on the Mac, but we were unable to figure out how to load PCD images from the hard drive into Photoshop on the Mac. This is apparently a known problem with Photoshop versions after 4.0. If anyone wants to lend a hand, please check out the Experiments page.

Updates

A fellow PhotoCD hacker on the 'Net has pointed out that the PhotoCD to RGB conversion can produce a very wide range of numbers. Much larger than Kodak indicates. This means that there may be more to all of this. Since I no longer use Photo CDs, I'll be unable to persue this, but if anyone else does, let me know and I'll post your results.