Oyranos/Use Cases
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Introduction
Oyranos provides many settings, which are not easily to understand without colour management background. The following Use Cases will describe practical questions and they're academic answer by Oyranos.
Home user browser/viewer/print application
The user needs some way to copy files from the camera. The application, lets call it SmartBrowse (SB), must check if a ICC profile is embedded or if the colorimetric parameters of the colour space are otherwise specified in the image. This is the image colour space.
In detail this is:
- the embedded profile, if it is not available ask for
- the EXIF tag and look at the colour space, or
- other colorimetric information (e.g. Tiff IFD or OpenEXR headers with colorimetric parts)
If SB cant find the desired information in the image it becomes a bit uncertain.
The next is to query a hardware database. If SB has no information about the camera from an EXIF tag, but EXIF is pretty standard, it should include such information into the loaded file. In TIFF the model, make and software tags are good candidates to store such information or add an EXIF tag, with at least what is available from loading the images.
SB should as well be able to look into a common hardware database for comparing the camera model and possibly serial number with available ICC profiles for the system, ahhemmm but it is not yet ready in Oyranos.
In case no profile is returned, SB will ask Oyranos what to do next. To use Oyranos we have to prepare some bits.
Code Preparation
SB has to put a
#include <oyranos.h>
in the code, in order to use Oyranos. oyranos-config --cflags delivers the compiler flags and oyranos-config --ldflags the linker flags. Or use pkg-config if installed.
You can then link Oyranos into your application with:
cc `oyranos-config --cflags` `oyranos-config --ldflags` mycode.c -o SB
Behaviour Preferences
As most tasks on the computer make sense by automating many steps to ease life of the user, Oyranos provides some settings, which will route SB to do its work automatically right. We may ask: what did the user decide to do in the case of getting untagged data? ... and write:
int untagged_action = oyGetBehaviour (oyBEHAVIOUR_ACTION_UNTAGGED_ASSIGN );
With a 0 in untagged_action nothing needs to be done. The image shall not be colour corrected further.
With a 1 in untagged_action the image shall obtain a standard colour space. We will continue with this case below.
The third possibility comes with a 2 in untagged_action. It means a dialog shall popup to let the user select a appropriate profile by himself.
Standard Profile Preferences
Then SB can ask Oyranos for its default RGB profile setting. The call does this:
char *profile_name = oyGetDefaultProfileName ( oyASSUMED_RGB , myAllocateFunc );
Obtaining a Profile
After obtaining the profile name it can be loaded by:
size_t size; void *standard_rgb_profile = oyGetProfileBlock (profile_name, &size, myAllocateFunc);
The next step is to query Oyranos for the display profile as SB want to show immediately thumbnail images. Here SB source text can include
#include <oyranos/oyranos_monitor.h>
char *monitor_profile = oyGetMonitorProfile (display_name, &size, myAllocateFunc);
should deliver the needed bits.
Some important options to pass to your ColourMatchingModuls are queried now:
Rendering Intent, Black Point Compensation, possibly a third profile — the proofing profile with its own rendering intent and the option for marking out of gamut colours.
Neutral gray is recommended as the out of gamut colour. Areas marked with gray show a good contrast to any strong saturated colour, which may fall out of gamut.
That's all information you need to correct the image for displaying on monitor.
Printing
For printing in a very simple Home user setup, we see just one printer and can use the proofing profile as the one default print device profile. That's very simple. Ask for the proofing profile and convert the image from the image colour space to the proofing colour space with the default intent. Sent the data to the printer driver or create a print file.
Image editor
In version 2.0 of SB the capability to edit and save images is introduced. The new application should behave on loading images the same as before regarding colour management.
Less is More
At time of been asked for editing the image, the original colour space may be exchanged depending on options in Oyranos. This late conversion shall ensure no unnecessary conversion is done. Marti Maria, the author of the LittleCMS color engine, likes to point out that colour transforms are a lossy action, which can, while happen often, decrease the quality. This should be expected especially for 8-bit per colour channel representations.
Editing Colour Space Concept
What is needed is to circumvent the native device colour spaces. They are often bumby and have strange gray behaviour. Thats' what the device profiles are for — to eliminate such unevenness in a devices colour space.
For editing the gray level should stay equal if changing for instance in RGB all tree channels at the same factor. Therefore in Oyranos exist beside the Assumed Profiles the Editing Profiles.
All behaves in SB v2.0 the same as before. But when the user wants to adjust some curves in the new SB he will probably do it on in the Editing colour space. The Editing colour space is as well known as workspace.
The appropriate profile is queried with:
char *profile_name = oyGetDefaultProfileName ( oyEDITING_RGB , myAllocateFunc );
for editing in Rgb.
Convert on Mismatch Behaviour
SB should now compare the image colour space, as described in the previous SB version, with the Editing colour space. If they are equal all is ok and editing can begin. If not the oyBEHAVIOUR_ACTION_OPEN_MISMATCH_RGB behaviour should be asked what to do.
Obtaining 0 means no conversion shall occur and the image profile remains unchanged.
1 means the conversion to the Editing RGB Colour Space shall be done automatically without asking further.
2 means pop-up a dialog for presenting alternative colour spaces for converting to. The oyEDITING_RGB can be preselected.
For the choices 1 and 2 SB needs to convert to the corresponding colour spaces. For the dialog SB can either integrate the usual rendering options (oyBEHAVIOUR_RENDERING_INTENT, oyBEHAVIOUR_RENDERING_BPC) with the Oyranos defaults preselected or just use them without further question.
The image is now ready for editing.
Afterwards the edited image can be shown on screen and printed as in the pure viewer version of SB.
