Drawing a graphic with a transparent color using xlib

Question

Using Xlib: I'm trying to display a pixmap with a transparent colour (color). That is a colour that will show the background. Currently drawing the pixmap completely opaque using XCopyArea (). How can I make this transparent (to work like XDrawString)?

Thanks

    GC pixgc = XCreateGC (XDisplay, pix, 0, NULL);
    XSetForeground (XDisplay, pixgc, ColourToXColour (RGB (0xff, 0xFF, 0xff)));
    XFillRectangle (XDisplay, pix, pixgc, 0, 0, 32, 32);
    XSetForeground (XDisplay, pixgc, ColourToXColour (RGB (0x00, 0x80, 0x80)));
    XDrawLine (XDisplay, pix, pixgc, 8, 8, 8 + 16, 8 + 16);
    XDrawLine (XDisplay, pix, pixgc, 8, 8 + 16, 8 + 16, 8);
    XDrawArc (XDisplay, pix, pixgc, 6, 6, 20, 20, 0, 270 * 64);
    XCopyArea (XDisplay, pix, Res->XWindow, DefaultGC (XDisplay, XScreen), 0, 0, 32, 32,  10, 50);

Hey man. Sorry I don't know the answer to your question. I edited the title a bit to distinguish your case from a case that requires a graphics card. — Joshua, Aug 11 '21 at 02:49

score 1 · Answer 1 · answered Aug 12 '21 at 22:28

Here is one possible solution. I would value better answers if any are possible.

// Mask is Black & White. Areas in Source corresponding White areas in Mask are drawn on Dest
bool CopyArea (Drawable Source, Drawable Dest, Drawable Mask, GC gc, int x, int y, int Width, int Height, int DestX, int DestY)
  {
    int a, b;
    XGCValues gcv;
    int func;
    Pixmap Mask_;
    //
    // Clear effected pixels in Dest
    XGetGCValues (XDisplay, gc, GCFunction, &gcv);
    func = gcv.function;
    gcv.function = GXandInverted;
    XChangeGC (XDisplay, gc, GCFunction, &gcv);
    XCopyArea (XDisplay, Mask, Dest, gc, x, y, Width, Height, DestX, DestY);
    // Generate coloured Mask to OR into Dest
    Mask_ = XCreatePixmap (XDisplay, Dest, Width, Height, 24);   // First make a copy of Mask
    gcv.function = GXcopy;
    XChangeGC (XDisplay, gc, GCFunction, &gcv);
    XCopyArea (XDisplay, Mask, Mask_, gc, x, y, Width, Height, 0, 0);
    gcv.function = GXand;   // Then colour it to match pixels in Source
    XChangeGC (XDisplay, gc, GCFunction, &gcv);
    XCopyArea (XDisplay, Source, Mask_, gc, x, y, Width, Height, 0, 0);
    // Copy new coloured Mask into Dest
    gcv.function = GXor;
    XChangeGC (XDisplay, gc, GCFunction, &gcv);
    XCopyArea (XDisplay, Mask_, Dest, gc, 0, 0, Width, Height, DestX, DestY);
    // Restore gc & free new Mask
    gcv.function = func;
    XChangeGC (XDisplay, gc, GCFunction, &gcv);
    XFreePixmap (XDisplay, Mask_);
  }

The call to XCreatePixmap: I have replaced the 24 with a global XDepth, which is derived such: XDisplay = XOpenDisplay (NULL); XScreen = DefaultScreen (XDisplay); XDepth = DefaultDepth (XDisplay, XScreen); — Stewart Tunbridge, Aug 18 '21 at 06:38

Drawing a graphic with a transparent color using xlib

1 Answers1