Older blog entries for Raphael (starting at number 43)

Mapping JPEG compression levels between Adobe Photoshop and GIMP 2.4

Some GIMP users who follow tutorials written for Adobe Photoshop are sometimes confused when they see statements like “Save your image using quality 8 or 9 in order to get good results” because this obviously does not match the scale from 0 to 100 used by GIMP (and other software based on the IJG JPEG library).

While working on some improvements for GIMP’s JPEG plug-in, I investigated the compression levels used by various programs and cameras. The analysis of several sample images allowed me to build the following mapping table (slightly updated since I posted a similar message to the gimp-web mailing list in August):

• Photoshop quality 12 <= GIMP quality 98, subsampling 1×1,1×1,1×1
• Photoshop quality 11 <= GIMP quality 96, subsampling 1×1,1×1,1×1
• Photoshop quality 10 <= GIMP quality 93, subsampling 1×1,1×1,1×1
• Photoshop quality 9 <= GIMP quality 92, subsampling 1×1,1×1,1×1
• Photoshop quality 8 <= GIMP quality 91, subsampling 1×1,1×1,1×1
• Photoshop quality 7 <= GIMP quality 90, subsampling 1×1,1×1,1×1
• Photoshop quality 6 <= GIMP quality 91, subsampling 2×2,1×1,1×1
• Photoshop quality 5 <= GIMP quality 90, subsampling 2×2,1×1,1×1
• Photoshop quality 4 <= GIMP quality 89, subsampling 2×2,1×1,1×1
• Photoshop quality 3 <= GIMP quality 89, subsampling 2×2,1×1,1×1
• Photoshop quality 2 <= GIMP quality 87, subsampling 2×2,1×1,1×1
• Photoshop quality 1 <= GIMP quality 86, subsampling 2×2,1×1,1×1
• Photoshop quality 0 <= GIMP quality 85, subsampling 2×2,1×1,1×1

The quality settings in Adobe Photoshop include not only the compression factor that influences the quantization tables, but also the type of chroma subsampling performed on the image. The higher quality levels use no subsampling, while the lower ones use 2×2 subsampling. The strange transition between Photoshop quality 6 and 7 (quality 6 having a higher equivalent IJG quality than 7) can be explained by the difference in subsampling: since quality 6 has less color information to encode, the size of the file will be smaller anyway, even if more coefficients are preserved in the quantization step.

You may also be surprised by the fact that the default GIMP JPEG quality level (85) matches the lowest quality offered by Photoshop: quality 0. This makes sense if you consider that the default “Save” offered by Photoshop is designed for high-quality images, so the losses should be minimized. But if you want to save images for web publishing, then Photoshop has a separate “Save for Web” feature that can save images using lower quality levels:

• Photoshop save for web 100 <= GIMP quality 98, subsampling 1×1,1×1,1×1
• Photoshop save for web 90 <= GIMP quality 96, subsampling 1×1,1×1,1×1
• Photoshop save for web 80 <= GIMP quality 93, subsampling 1×1,1×1,1×1
• Photoshop save for web 70 <= GIMP quality 90, subsampling 1×1,1×1,1×1
• Photoshop save for web 60 <= GIMP quality 85, subsampling 1×1,1×1,1×1
• Photoshop save for web 50 <= GIMP quality 86, subsampling 2×2,1×1,1×1
• Photoshop save for web 40 <= GIMP quality 79, subsampling 2×2,1×1,1×1
• Photoshop save for web 30 <= GIMP quality 74, subsampling 2×2,1×1,1×1
• Photoshop save for web 20 <= GIMP quality 70, subsampling 2×2,1×1,1×1
• Photoshop save for web 10 <= GIMP quality 60, subsampling 2×2,1×1,1×1

This mapping between Photoshop and GIMP quality levels for JPEG is not exact and is intentionally pessimistic for GIMP. There is some safety margin, so it is possible to decrease the GIMP quality level a bit and still get a file that is as good as the one saved by Photoshop.

Reminder: if you think that you will need to re-edit an image later, then you should never save it only in JPEG format. Always keep a copy in XCF format (GIMP’s native file format) so that you can edit it without losing any additional information.

Another reminder: using a JPEG quality level below 50 or above 98 is not a good idea. Saving a JPEG image using quality 99 or 100 is just a waste of disk space. You should use a different file format instead (such as PNG or TIFF). And below quality 50, you lose so much that it would be better to rescale your image and use a lower resolution before trying to compress it further.

Syndicated 2007-10-23 16:11:51 from Raphaël's Last Minutes

Stupid error message

Last Wednesday, I went to the gas station because my car was a bit thirsty. When I wanted to insert my card and pay for the fuel, I was greeted with this ridiculous error message: “The exception unknown software exception (0×0eedfade) occurred in the application at location 0×77e73887.”

This is so wrong..

• The error message goes to the wrong target: the customer cannot do anything about it anyway, so why does it appear on the screen? The touch screen was frozen so I could not even press the OK button. In cases like this, the software should just log the error and blank the screen or display some customer-oriented error message such as “Out of order”. There should be a way to trap these errors (any kind of software error) and redirect them to the company that maintains these terminals instead throwing them at the customer.
• The exception “unknown software exception” shows that things are definitely not under control. How can one trust a system that displays such a stupid error message?
• Minor detail: the error message is in English only, while the user interface of this terminal defaults to French and supports multiple languages (Dutch and German, but not English). Trapping the error and displaying “Out of order” in multiple languages would have been more appropriate and more customer-friendly.
• If you ask Google about this error message by searching for the error code and address, you will find several matches revealing that various applications are affected by this random crash: Internet Explorer, Photoshop, some Delphi applications and other specialized software. This looks like a mysterious Windows crash that confuses everybody.
• Using Windows instead of a more robust embedded operating system is just asking for trouble. The main advantage may be that some customers are already familiar with the Windows error dialogs and can recognize them from a distance, so they know that they should go away and not even bother reading the error message.

Syndicated 2007-10-22 08:51:57 from Raphaël's Last Minutes

Surprise on the way to the airport

Last month, I was driving a bit fast towards Brussels because I didn’t want to miss my plane. It was raining heavily, but fortunately there wasn’t too much traffic on the motorway.

Suddenly, I see the three cars in front of me slowing down very quickly and switching from the first to the third lane. Oops! The road is covered with sugar beets and some of them are still rolling… Fortunately, I was paying attention so I quickly stepped on the brakes and then moved to the third lane like the other cars. Once I had slowed down enough, I grabbed my mobile phone and took this photo through the windshield without aiming much because I still had to be careful and avoid the other cars… not to mention slaloming to avoid the rogue beets trying to attack my car.

That was an interesting driving experience… I eventually reached the airport safely and got my plane just in time.

Syndicated 2007-10-21 22:56:08 from Raphaël's Last Minutes

Tivoization still possible with GPLv3 (draft4)?

The latest draft of the GPLv3 contains many improvements over the previous ones. It also still contains several minor issues, some of which date back to the first draft. Among these, there is a paragraph that remained unchanged since the first draft, although there were several comments saying that it could provide a loophole:

The Corresponding Source need not include anything that users can regenerate automatically from other parts of the Corresponding Source.

The problem is that “automatically” is not defined and it could lead to abuses, including preventing users from running modified versions of GPL software on some devices (the Tivoization problem that GPLv3 tries to prevent). “Automatically” can cover current practices such as generating Makefile from Makefile.in using autoconf, generating parser.c from parser.y using bison, etc.

But “automatically” could also include some operations that are impractical in terms of time or special equipment required. A file that can be regenerated automatically but requires several hundred years of computation on a supercomputer will effectively prevent most people from compiling the software and installing it on their device (if that file is required during installation or during run time). The canonical example would be if the tool that regenerates the missing source file requires the factorization of the product of two very large prime numbers.

As long as the company selling the device provides the complete Corresponding Source (including tools necessary for regenerating the missing files) and Installation Information, then they would be compliant to the GPLv3. As long as the source code (with the missing file) is the “prefered form of the work for making modifications to it”, then they have followed the GPL to the letter… while still preventing users from running modified code on their devices.

Of course I reported this problem and I included links to the previous comments on the same issue. But it looks like this issue has been ignored so far, despite the fact that the comments on the first draft are more than a year old.

Syndicated 2007-06-20 20:25:10 from Raphaël's Last Minutes

Customer service

I bought my current camera (Nikon D70) in May 2004, just a few days after that model started to appear in shops. I made several thousand photos with this camera and I have been happy with the results… Until a few months ago, when it started behaving in a strange way: sometimes the camera would take unusually long to power up, sometimes it would appear to work correctly for a while but then suddenly start to display bogus exposure values. Without moving the camera or changing the lighting conditions, it would randomly change between 1/500s and 2 seconds, or some other very different values. When the camera started to behave like that, it was almost impossible to shoot anything with the correct exposure.

Over time, this problem became more and more severe so I eventually decided to bring the camera back to the shop for repair. My camera was 3 years old, so the warranty had already expired. The guy at the shop asked me to pay 50 EUR in advance and told me that he would send it to Nikon for repair, without expecting too much: regardless of the brand and model, many cameras are just returned because repairing them would cost more than a new model. This is a bit annoying when the camera costs around 1000 EUR.

Two weeks later, I got a letter telling me that my camera was back in the shop. The letter did not mention any repairs, so I was a bit worried. I went to the shop to pick it up and I had a pleasant surprise: Nikon replaced the defective circuits at no cost. When I tested the camera, I found that they also upgraded the firmware to a newer version, which includes new features and fixes some bugs that I had encountered before. And they also cleaned up the camera (inside and outside). So this is a happy end. Thanks to the Nikon customer service!

Syndicated 2007-05-14 14:46:52 from Raphaël's Last Minutes

Sophie

Sophie was born 6th of September, a bit before 16:00. She is not a small baby, weighting 4.390 kg and measuring 55 cm, but she did not take too long to come out and greet her happy parents Isabelle and Raphaël. A couple of hours later, she met her sister Catherine who is now two years old. The baby and mother are doing fine. The main job of the father was to hold the mother’s hand. This very important job was accomplished successfully according to the mother.

The picture below shows Sophie less than 4 hours after her birth. She was about to ask for more food…

Syndicated 2006-09-07 09:17:57 from Raphaël's Last Minutes

Print shops and posters

At the end of last year, my wife Isabelle asked me if I could create some postcards for the association she works for. The idea was to have something that can serve both as a greeting card for the new year and an indication that they have recently moved into new offices. After a couple of iterations with POV-Ray and GIMP, I eventually managed to create something that she and her colleagues were happy with. I created a winter scene showing their front door (modelled from photos), some cardboard boxes in front of it and some trees in the background instead of the ugly wall that is there in reality.

Later, she asked me to create some posters showing different versions of this scene. I re-rendered the winter scene at a higher resolution (approx. 3000 * 2000) and I also created a summer scene in which the snow is gone, the grass is greener and there are leaves on the trees. Here are some smaller versions of these images:

I gave Isabelle a CD-ROM containing these images and she went to the cheapest print shop nearby to get them printed on posters (100 * 70cm). At that size, the resolution of the images is 75dpi. I thought that it would be good enough for a poster that is designed to be viewed from some distance.

Well, it turns out that the employee of that shop discouraged Isabelle from printing the posters because they were not at 300dpi. She claimed that the images would look bad because of the low resolution. On the other hand, she looked at the samples that I had given to my wife (printed on photo paper) and she said that she would not be able to get the same quality on posters. Go figure. Oh, and she also said that her computer could not read that strange PNG format, but fortunately I had also included a JPEG version.

Anyway, she asked for 300dpi so she will get them. For 100 * 70cm, 300dpi means about 12000 * 8000 pixels, for a total of 97 megapixels. I wish I had a camera that could take photos at that resolution. I estimated that re-rendering the image with POV-Ray would take a couple of weeks using my fastest computer (which just died anyway - see my previous entry), so I simply let GIMP scale up the image and I only recreated the parts in which the details could be interesting. In the process, GIMP asked me if I really wanted to create an image that took more than 1 GB. Well, sure, that’s what the shop asked for! I will burn these files on a CD now. I hope that they will not complain that the image is too large for their computer to handle…

Syndicated 2006-01-16 07:48:59 from Raphaël's Last Minutes

Dead PC

Back from vacation. Found that my PC does not want to start anymore. What a nice way to start the new year!

I did not have much time to investigate what died. It could be the motherboard, the CPU, the RAM, the video card or the power supply. In any case, the PC locks up on startup and does not seem to initialize the PCI cards correctly. I am guessing that because I get no video signal and the lights on the network card get stuck in an unusual state, which usually means that the card did not get the proper signals on the PCI bus. Regardless of what it is, it looks like I will have to waste some time finding the bad components and replacing them.

Fortunately, my data should be safe. I learned a lot from the disk crash that I experienced a few years ago just when I was starting my (then irregular) backup on CDs. At that time, that caused me to lose a number of GIMP source files that I was working on, such as a preliminary version of a GIMP macro recorder. Since then, I have started doing automated backups on external USB drives in addition to the less frequent manual backups on CDs. USB enclosures are relatively cheap (about 10 EUR) and buying two hard disks instead of one (or 4 instead of 2) is also cheap in comparison with the value of the data that could be lost.

Even if my main PC will not be usable for the next few days, I can still use the laptop that I got from work. But for how long? I just learned that 2 of my 6 colleagues who had the same model of laptop experienced a hard disk crash in the last two weeks. Another one had a disk crash a few weeks before the Christmas break. And another two lost their data after coming back from vacation due to software problems. So this means that 5 out of 6 had problems with their laptop recently (OK, in two cases this was likely to be a human error, but still…). I am starting to wonder if there is a timebomb built into these laptops. So maybe I am next on the list. I’m going to back up my laptop now…

Syndicated 2006-01-16 05:30:37 from Raphaël's Last Minutes

Features and remote controls

Jimmac posted a very interesting journal entry about “more features” and a comparison with remote controls. I thought about it a bit and started typing a comment to his blog, but as it grew longer and longer I decided to move it here so that I have a bit more space…

This nice comparison with a remote control made me think about how I use mine. The device that controls my TV (and VCR) has a little flip hiding the “advanced” buttons. I do not have a photo of my remote control, but I found one that is reasonably similar. The top part until the red power button can be opened to reveal more buttons.

The basic buttons such as channel selection, volume and start/stop for the VCR are always visible. If you lift the flip, you get extra buttons for selecting input sources (e.g., DVD player or camera) or outputs, changing the parameters of the display and other exotic stuff. As it turns out, two of the “advanced” buttons on my remote control are as worn out as the channel and volume selection buttons that are among the “normal” buttons. So I am very glad that these buttons exist, otherwise the device would have been far less useful to me. On the other hand, I understand that they are hidden under a flip because some other people (whether they are a majority or a minority is irrelevant) would probably never use them. Even myself, I do not need to see all buttons all the time.

The comparison with this little device that I use almost every day is very interesting to me. And I take some lessons from it: the designers of this device could not predict which buttons I would use most frequently. They modeled this device for a slightly different class of users, yet I enjoy using it because the “advanced” buttons that I need are still within reach when I need them.

In case you cannot guess where I’m heading yet, let me add that the “advanced” buttons are not visible by default so that they do not confuse those who do not need them, but they are easily discoverable and once you get to them, they look and behave exactly like the other buttons. I do not need to open my TV and flip a few DIP switches to get to the advanced options. Translated in the GNOME or GIMP world, this means that I would have a default set of features that I can play with but also some advanced settings that are easily accessible from the application without having to resort to some other obscure tool such as a gconf editor.

I do not really need to have “everything at my fingertips” and be confused by huge configuration dialogs for every little feature. In fact, I prefer to have the less frequently used options hidden away, as long as there is an easy way to access them whenever I need them. It is essential for these advanced options and features to be discoverable, so the dialog that contains the basic options should give a visible hint that more stuff exists but is hidden (using an expander for the dialog or an “advanced options” button or whatever).

Also, even if I do not use some of the other “advanced” buttons on the remote control, I do not think that I would have bought that TV set if I had not seen that these options were available.

Update: moved the image and its description up a bit.

Syndicated 2005-12-14 09:18:15 from Raphaël's Last Minutes

Resurrecting GIMP metadata

It has been a long time since I last updated the metadata code in GIMP CVS. It is annoying how real life gets in the way… In fact, my contributions to the GIMP have been almost nonexistant in the last months, except for some minor contributions to the help pages. But I will take advantage of the Christmas break to get back to that metadata code.

First I have to fix some outstanding issues with the model (based on XMP) so that it is easier to link the model and the GUI. Currently, opening the File->Properties window leads to an ugly collection of empty widgets. Once the model and the GUI are correctly linked, it should be possible to have correct values displayed in these widgets and to update the model when the values are edited by the user.

Once this first step is done, it should be much easier to work on the remaining parts: adding pretty widgets in the properties window, converting to and from EXIF and IPTC, and eventually moving some of the code currently residing in gimp/plug-ins/metadata/ into a library so that plug-ins could link this code directly instead of using some functions through the PDB. I will write more about this once the first step is done.

Syndicated 2005-12-07 04:20:35 from Raphaël's Last Minutes