[directfb-users] How to make IDirectFBSurface::blit supporting a transparent background?

[Kelvin Chen]

Hi, all:

How can I make IDirectFBSurface::blit supporting a transparent background? I
have tried a lot of methods, but it doesn't work.

Here is my problem.
1> draw something to the primary_surface
2> create a tempory_surface with transparent background and draw something
to it
3> blit the content of temporary_surface to the pramary_surface, but this
operation should not erase what primary_surface already have

Here is my code, it doesn't work, I don't know why. The blue rectangle draw
on the primary_surface will be erased after the blit operation.

Thanks a lot!

#include <iostream>
#include <directfb.h>
*using* *namespace* std;

*int* main(*int* argc, *char* *argv[])
{
    IDirectFB *dfb;

    // Initialize DirectFB
    DirectFBInit(&argc, &argv);
    DirectFBCreate(&dfb);
    dfb->SetCooperativeLevel(dfb, DFSCL_FULLSCREEN);

    // Create primary surface
    IDirectFBSurface *primary_surface;
    DFBSurfaceDescription primary_surface_description;
    primary_surface_description.flags =
DFBSurfaceDescriptionFlags(DSDESC_CAPS | DSDESC_WIDTH | DSDESC_HEIGHT
| DSDESC_PIXELFORMAT);
    primary_surface_description.caps =
DFBSurfaceCapabilities(DSCAPS_PRIMARY | DSCAPS_FLIPPING);
    primary_surface_description.pixelformat = DSPF_ARGB;
    primary_surface_description.width = 1280;
    primary_surface_description.height = 800;
    dfb->CreateSurface(dfb, &primary_surface_description, &primary_surface);
    // Draw a big blue square
    primary_surface->SetColor(primary_surface, 0x00, 0x00, 0xff, 0xff);
    primary_surface->FillRectangle(primary_surface, 50, 50, 600, 600);
    primary_surface->Flip(primary_surface, NULL, DFBSurfaceFlipFlags(0));
    // Sleep 1 seconds to observe the result
    sleep(1);

    // Create temporary surface
    IDirectFBSurface *temporary_surface;
    DFBSurfaceDescription temporary_surface_description;
    temporary_surface_description.flags =
DFBSurfaceDescriptionFlags(DSDESC_CAPS | DSDESC_WIDTH | DSDESC_HEIGHT
| DSDESC_PIXELFORMAT);
    temporary_surface_description.caps =
DFBSurfaceCapabilities(DSCAPS_FLIPPING); // Not primary
    temporary_surface_description.pixelformat = DSPF_ARGB;
    temporary_surface_description.width = 1280;
    temporary_surface_description.height = 800;
    dfb->CreateSurface(dfb, &temporary_surface_description, &temporary_surface);
    // Clear the background with transparent black
    temporary_surface->Clear(temporary_surface, 0, 0, 0, 0);
    temporary_surface->SetColor(temporary_surface, 0, 0, 0, 0);
    temporary_surface->FillRectangle(temporary_surface, 0, 0, 1280, 800);
    // Draw a small red square
    temporary_surface->SetColor(temporary_surface, 0xff, 0x00, 0x00, 0xff);
    temporary_surface->FillRectangle(temporary_surface, 0, 0, 200, 200);
    temporary_surface->Flip(temporary_surface, NULL, DFBSurfaceFlipFlags(0));
    // Copy the content of the temporary surface to primary surface
    DFBRectangle rect;
    rect.x = 0;
    rect.y = 0;
    rect.w = 1280;
    rect.h = 800;
    temporary_surface->SetSrcBlendFunction(temporary_surface, DSBF_SRCALPHA);
    primary_surface->SetDstBlendFunction(primary_surface, DSBF_DESTALPHA);
    primary_surface->Blit(primary_surface, temporary_surface, &rect, 0, 0);
    primary_surface->Flip(primary_surface, NULL, DFBSurfaceFlipFlags(0));
    // Sleep 3 seconds to observe the result
    // Note that the original blue square is not destroyed
    sleep(3);

    temporary_surface->Release(temporary_surface);
    primary_surface->Release(primary_surface);
    dfb->Release(dfb);

    *return* 0;
}




-- 
Kelvin Chen

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