savepng and transparency

Johan Halmén

I want to create a png file with alpha transparency. I've downloaded loadpng and it loads png files ok, but when I try to save a BITMAP, I get a rgba image with nothing in the channels. How should I create the BITMAP in my Allegro app and how should I use the save functions in loadpng to get what I want.

My application is an update of my Eggifyer, which reads an image and mapps it on an egg. Now I want a transparent surrounding for the egg and a semi-transparent shadow that fades to the edges. All that looks fine in the application on my screen, but I want it saved so that I could place the egg on a web site.

Peter Wang

Have you made sure the image is 32-bit? I don't remember whether it works but it looks like I put in code to specifically deal with 32-bit BITMAPs.

Johan Halmén

Well I think it is 32 bit. I do set_color_depth(32), then I do set_gfx_mode(). After that each create_bitmap() should create a 32 bit bitmap, right? The saved png has four channels but the alpha is black. I managed to separate the channels, revealing the image in the rgb channels. I guess my problem is drawing the image in the first place. I've been RTFM, but I haven't figured out when the drawing functions actually change the alpha value for a pixel in the bitmap.

I draw the image pixel by pixel. For the background I imagine I could do:

a = 0;
col = makeacol32(r, g, b, a);
putpixel(bmp, x, y, col);

and for the object I have methods for calculating r, g, b and a. a is 255 except for the edge, where I interpolate to get rid of crispiness, kind of anti-aliasing.

But the result looks like if a would calculate to 0 for the whole image.

Peter Wang

Allegro's alpha support is pretty... idiosyncratic. I managed to get the following program to work though. The trick was to set_color_depth(32); Otherwise you might look into set_write_alpha_blender().

1#include <allegro.h>
2#include <loadpng.h>
3 
4int main(void)
5{
  BITMAP *bmp;
  int x, y;
  allegro_init();
  set_color_depth(32);
  set_color_conversion(COLORCONV_NONE);
  bmp = create_bitmap_ex(32, 255, 255);
  for (y = 0; y < 256; y++) {
      for (x = 0; x < 256; x++) {
          int r = x;
          int g = y;
          int b = x;
          int a = y;
          int c = makeacol(r, g, b, a);
          putpixel(bmp, x, y, c);
      }
  }
  save_png("test.png", bmp, NULL);
  destroy_bitmap(bmp);
  return 0;
25}

Johan Halmén

Thanks for that!

Thread #577955. Printed from Allegro.cc

1	#include <allegro.h>
2	#include <loadpng.h>
3
4	int main(void)
5	{
6	BITMAP *bmp;
7	int x, y;
8	allegro_init();
9	set_color_depth(32);
10	set_color_conversion(COLORCONV_NONE);
11	bmp = create_bitmap_ex(32, 255, 255);
12	for (y = 0; y < 256; y++) {
13	for (x = 0; x < 256; x++) {
14	int r = x;
15	int g = y;
16	int b = x;
17	int a = y;
18	int c = makeacol(r, g, b, a);
19	putpixel(bmp, x, y, c);
20	}
21	}
22	save_png("test.png", bmp, NULL);
23	destroy_bitmap(bmp);
24	return 0;
25	}