Archive for October, 2007

Cyclomatic complexity in GIMP code

Thursday, October 25th, 2007

After reading Xan’s article The Cyclomatic Horror From Outer Space analyzing the complexity of some GTK functions, I was curious and I wanted to run the same test in the GIMP source tree in order to see what parts of the code would be the hardest to test. This test is very simple and can be summarized as counting the number of decision points in every function in a program (so you get an idea of the number of possible code paths).

I did as suggested and I started with “apt-get install pmccabe“, followed by “pmccabe app/*/*.c | sort -nr | head -10” to get the 10 functions with the highest (worst) results. This gave me the following table:

Cyclomatic complexity Lines of code Function name
113 892 gimp_display_shell_canvas_tool_events
100 123 layers_actions_update
69 416 update_box_rgb
61 359 border_region
60 445 siox_foreground_extract
56 452 render_image_tile_fault
54 327 combine_inten_a_and_inten_a_pixels
53 194 gimp_plug_in_procedure_add_menu_path
47 275 gimp_drawable_offset
46 240 gimp_vector_tool_oper_update

According to the CMU page on cyclomatic complexity, numbers between 21 and 50 reveal a “complex, high risk program” and numbers above 50 only occur in an “untestable program (very high risk)“.

What does this mean for GIMP? Not much. But if you touch one of these functions, please be careful… you might break things and it will be very hard to find where the bugs are hiding in that code.

Mapping JPEG compression levels between Adobe Photoshop and GIMP 2.4

Tuesday, October 23rd, 2007

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.

Stupid error message

Monday, October 22nd, 2007

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 (0x0eedfade) occurred in the application at location 0x77e73887.

Reliability?

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.

Surprise on the way to the airport

Sunday, October 21st, 2007

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.

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