Hi all,

I'm trying to use Allegro 5.2.6 on Ubuntu 20.10
Here's a test case showing the problem I'm having.
When using a memory bitmap for a background image.
If I draw a sprite on top of it, I'm getting black boxes where I should
be seeing the background color.
It also seems to affect drawing primitives.
see attachment.

Everything works fine, if I use a video bitmap for background image.
But I wanted to use a memory bitmap because I'm going to do lots of drawing to it.

What am I doing wrong?

Thanks,
all.

DanielH,
Apologies, I should have attached the test.png file.
I updated my original post,

1.
you can see that test.png does have transparency for the background.

2.
And even if the test.png was broken (it's not), that does not explain the black boxes when drawing the line.

to clarify:
if I use a video bitmap for the background image there are no black regions.
It only happens when I use a memory bitmap.

Thanks.

Yes and also this clear command also.

al_clear_to_color(al_map_rgb(0,0,0));
al_draw_bitmap(bg, 0, 0, 0);

To your problem, I'm not sure. I'm not at home to test it.

What happens when you remove or comment out the draw line?
Do you still get the black under sprite?

I cannot open your "test.png", I get an error.

Could you try the one I have attached, just for testing? (No pun intended).

I have just tested using your original code and test.png (on Windows) and it works fine, must be a Ubuntu bug....

[EDIT]
Can you try to clear the display before drawing?

    al_set_target_backbuffer(display);
    al_clear_to_color(al_map_rgba(0,0,0,0));
    al_draw_scaled_bitmap(tmp, 0, 0, al_get_bitmap_width(tmp), al_get_bitmap_height(tmp), 0, 0, al_get_display_width(display), al_get_display_height(display), 0);
    al_flip_display();

I have tried using both DirectX and OpenGL.

Both work fine on Windows.

This looks like an Ubuntu specific bug; sorry I cannot be more help, but I don't have Ubuntu.

[EDIT]
One last try!! Maybe this?

The memory bitmap you are creating may not have an alpha channel.

Try setting the new bitmap format before creating it.

al_set_new_bitmap_format

Make sure you select a format with an alpha channel.

I checked on Debian (which has 5.2.4 packaged) and also with current git HEAD, both were OK.
Could be Ubuntu specific I suppose.

IIUC you are saying that merely drawing a memory bitmap (bg) to a display causes it to produce artifacts when you later draw primitives onto it.

What happens if you skip the 'tmp' part and draw directly to the window's display?
Also what if you al_save_bitmap the 'tmp' bitmap, is that also corrupted?

Looks like a driver bug, but I don't really get what the mechanism would be. When bg is drawn to tmp, tmp is locked READWRITE, and all the pixels are drawn in software. I don't have an explanation why the blocks only appear after the sprite and line are drawn. I'd be curious what the pixels in tmp look like after bg is drawn.

I'd avoid using memory bitmaps, and instead do al_lock_bitmap on bg before calling your `al_put_pixel`'s.