Extending The Effects Engine Report W9

Hello again,

As you know from the last report, I made the Bessel Equalizers configurable from preferences. Daniel noticed that the "Enable EQs" check box did not cause immediate change. I took a look at the code responsible for managing that box and found out that was the desired behaviour. Only after clicking "Apply" the equalizers were enabled or disabled. I talked with my mentor and concluded that instant change would be nice. All I had to do was call DlgPrefEQ::slotApply method inside DlgPrefEQ::slotEnaEQChanged. Also, on the EQ part I changed the order of the Bandpass Bessel filter to 4.

I started working at LV2 native support. The first step was getting lilv library to work inside Mixxx. I installed it on my system using the package manager. In order to work, the library needed to be linked when compiling the source files, using -llilv-0 argument. I don't have much experience with SCons so I tried a naive way of linking lilv: find where SCons is storing its build parameters and append -llilv-0 to them. This did not work because it was passed at the beginning of the command. I learned that g++ is passing only one time over its arguments and it links the library if the name of a source file is encountered before the library argument. For example:

The -llilv-0 argument inside this command is useless:
g++ -llilv-0 source_file.cpp

This works as it should:
g++ source_file.cpp -llilv-0

After doing a little research about SCons, I found out that linking libraries is simply a matter of adding the name of the library to a list: build.env.Append(LIBS='lilv-0'). Before merging the LV2 integration we must decide if we want to keep lilv as an external dependency as it currently is, or bundle it together with Mixxx (see how fidlib is handled).

I created a new class LV2Manifest which job is to receive a LilvPlugin* and create an EffectManifest from it. This is useful because Mixxx is using this EffectManifest to create and update parameter knobs. Inside this class I iterated through plugin's control ports and created corresponding EffectParameters. Here I set the control hint to EffectManifestParameter::CONTROL_KNOB_LINEAR and check if the value hint should be set to VALUE_FLOAT or VALUE_INTEGRAL using the lilv_port_has_property function. The plugin's id, name and author fields are initialized too. This class features a m_isValid boolean field which is set to true if we support the plugin. Currently there are two cases when we set this field to false:

• when the plugin has required features (we can check this with lilv_plugin_get_required_features function)
• when the plugin doesn't have two audio inputs and two audio outputs, since Mixxx is handling stereo samples

I created a basic LV2 backend and a first version of an effect processor. I will work on improving them during the next week.

I did some tests with kn0ck0ut[1] plugin. I used Audacity, jalv and lv2proc as LV2 hosts. The results are the same: the plugin is only playing silence regardless of how I set its parameters. So I joined the #lv2 Freenode channel and asked for help, which came right away. Here[2] you can read the conversation. This kind of responsiveness is what I love about open source communities :D . When we conceived this GSoC project, our goal was performing vocal removal on the fly, using the before mentioned plugin. Luckily I found another[3] LV2 plugin pack which features a Karaoke effect. I'll do some tests with it and keep you posted!

See you soon,
Nicu Badescu

[1] - https://github.com/jeremysalwen/kn0ck0ut-LV2
[2] - https://gist.github.com/badescunicu/724b035396ed7bdd9ac8
[3] - https://github.com/swh/lv2
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