Note: original text by @Antonio Vazquez (antoniov)

Current Status

The current logic is using a general place to save the brush by “mode” (weight paint, texture paint, gpencil sculpt, etc). This information is currently saved in ToolSettings struct:

typedef struct ToolSettings {
    VPaint *vpaint;        /* vertex paint */
    VPaint *wpaint;        /* weight paint */
    Sculpt *sculpt;
    UvSculpt *uvsculpt;    /* uv smooth */
    GpPaint *gp_paint;
            ..
            ..

For each mode, we have a struct with the Paint data, for example:

typedef struct GpPaint {
    Paint paint;
} GpPaint;

And Paint struct contains the active brush.

/* Paint Tool Base */
typedef struct Paint {
    struct Brush *brush;
    struct Palette *palette;
            ..
            ..

When we need use a brush (in any mode), we usually have functions like this that uses ToolSettings as the source to find the corresponding paint.

Brush *BKE_brush_getactive_gpencil(ToolSettings *ts)
{
    /* error checking */
    if (ELEM(NULL, ts, ts->gp_paint)) {
        return NULL;
    }
    Paint *paint = &ts->gp_paint->paint;

    return paint->brush;
}

After looking at code, it’s clear that we cannot save what brush is active for each tool, so we need a place to save this.

I’m not sure what is the best place to do that, maybe add a Paint struct for each tool? add a bmain hash with tool->brush?

We also need a way on Python to get the active brush by tool to display its properties. Not sure, but we will need something like this, but by active tool (“active_tool_brush”??):

// Context function
if (CTX_data_equals(member, "active_gpencil_brush")) {
  Brush *brush = BKE_brush_getactive_gpencil(scene->toolsettings);
  if (brush) {
     CTX_data_pointer_set(result, &scene->id, &RNA_Brush, brush);
     return 1;
  }
}

Summary of what we need

• A way to save the brush of each tool.
• Functions to get the active brush for the current active tool.
• Python context functions to get the brush.
• Preview system for brushes (phase II).