Date: Wed, 2 Apr 2003 19:13:31 +0200
From: David Raulo <david.raulo@free.fr>
To: sdl@libsdl.org
Subject: Re: [SDL] SDL_Image patch for ILBM Amiga images

diff -BNaur sdl-image1.2-1.2.3/IMG_lbm.c sdl-image1.2-1.2.3.david/IMG_lbm.c
--- sdl-image1.2-1.2.3/IMG_lbm.c	2003-02-26 17:57:45.000000000 +0100
+++ sdl-image1.2-1.2.3.david/IMG_lbm.c	2003-04-02 18:48:42.000000000 +0200
@@ -84,7 +84,10 @@
 	Uint32      i, j, bytesperline, nbplanes, plane, h;
 	Uint32      remainingbytes;
 	Uint32      width;
-	BMHD	      bmhd;
+	BMHD        bmhd;
+	Uint32      camg;
+	Uint8       is_ham, is_ehb, red, green, blue;
+	Uint8       colors[8], ham_shift1, ham_shift2, ham_mask1, ham_mask2;
 	char        *error;
 
 	Image   = NULL;
@@ -131,9 +134,10 @@
 	}
 
 	nbcolors = 0;
-
+	camg = 0;
+	
 	memset( &bmhd, 0, sizeof( BMHD ) );
-
+	
 	while ( memcmp( id, "BODY", 4 ) != 0 )
 	{
 		if ( !SDL_RWread( src, id, 4, 1 ) ) 
@@ -183,6 +187,18 @@
 			nbcolors = size / 3;
 		}
 
+		if ( !memcmp( id, "CAMG", 4 ) ) /* Commodore AMiGa */
+		{
+			if ( !SDL_RWread( src, &camg, 4, 1 ) )
+			{
+				error="error reading CAMG chunk";
+				goto done;
+			}
+
+			bytesloaded = 4;
+			camg = SDL_SwapBE32( camg );
+		}
+
 		if ( memcmp( id, "BODY", 4 ) )
 		{
 			if ( size & 1 )	++size;  	/* padding ! */
@@ -206,7 +222,25 @@
 		nbplanes = 1;
 	}
 
-	if ( bmhd.mask ) ++nbplanes;       /* There is a mask ( 'stencil' ) */
+	if ( bmhd.mask & 0x01 ) ++nbplanes;     /* There is a mask ( 'stencil' ) */
+
+	/* Amiga HAM and EHB modes */
+
+	is_ham = 0;                        /* Hold And Modify */
+	is_ehb = 0;                        /* Extra Half-Brite */
+	if ( camg & 0x0800 )
+		is_ham = 1;
+	else if ( ( camg & 0x0080 ) ||
+			  ( ( nbplanes == 6 ) && ( nbcolors == 32 ) ) )
+		is_ehb = 1;
+	
+	if ( is_ham ) /* compute usefull values for decoding Amiga HAM images */
+	{
+		ham_shift1 = nbplanes - 2;
+		ham_shift2 = 10 - nbplanes;
+		ham_mask1 = ( 1 << ham_shift1 ) - 1;
+		ham_mask2 = ( 1 << ham_shift2 ) - 1;
+	}
 
 	/* Allocate memory for a temporary buffer ( used for
            decompression/deinterleaving ) */
@@ -217,13 +251,27 @@
 		goto done;
 	}
 
-	if ( ( Image = SDL_CreateRGBSurface( SDL_SWSURFACE, width, bmhd.h, bmhd.planes==24?24:8, 0, 0, 0, 0 ) ) == NULL )
-	   goto done;
+	/* Allocate a new SDL_surface */
+	
+	if ( (bmhd.planes==24) || is_ham )  /* 24bits, RGB format */
+		Image = SDL_CreateRGBSurface( SDL_SWSURFACE, width, bmhd.h, 24,
+#if SDL_BYTEORDER == SDL_LIL_ENDIAN
+									  0x0000FF, 0x00FF00, 0xFF0000,
+#else
+									  0xFF0000, 0x00FF00, 0x0000FF,
+#endif
+									  0 );
+	else
+		Image = SDL_CreateRGBSurface( SDL_SWSURFACE, width, bmhd.h,
+									  8, 0, 0, 0, 0 );
+	if ( Image == NULL )
+		goto done;
 
 	/* Update palette informations */
 
-	/* There is no palette in 24 bits ILBM file */
-	if ( nbcolors>0 )
+	/* There is no palette in 24 bits ILBM file, and Amiga HAM images are
+	   converted to 24 bits */
+	if ( nbcolors>0 && ! is_ham )
 	{
 		Image->format->palette->ncolors = nbcolors;
 
@@ -235,6 +283,22 @@
 			Image->format->palette->colors[i].g = *ptr++;
 			Image->format->palette->colors[i].b = *ptr++;
 		}
+
+		/* In Amiga "Extra Half-Brite" mode, the second half of the colormap
+		   is computed from the first half, with components divided by 2 */
+		if ( is_ehb )
+		{
+			Image->format->palette->ncolors = 64;
+
+			ptr = &colormap[0];
+
+			for ( i=0; i<32; i++ )
+			{
+				Image->format->palette->colors[i+32].r = *ptr++ >> 1;
+				Image->format->palette->colors[i+32].g = *ptr++ >> 1;
+				Image->format->palette->colors[i+32].b = *ptr++ >> 1;
+			}
+		}
 	}
 
 	/* Get the bitmap */
@@ -300,7 +364,7 @@
 		/* One line has been read, store it ! */
 
 		ptr = Image->pixels;
-		if ( nbplanes==24 )
+		if ( nbplanes==24 || is_ham )
 			ptr += h * width * 3;
 		else
 			ptr += h * width;
@@ -313,11 +377,18 @@
 		{
 			if ( nbplanes!=24 )
 			{
+				if ( is_ham )
+				{
+					red   = colormap[0];
+					green = colormap[1];
+					blue  = colormap[2];
+				}
+
 				size = ( width + 7 ) / 8;
 
 				for ( i=0; i < size; i++ )
 				{
-					memset( ptr, 0, 8 );
+					memset( colors, 0, 8 );
 
 					for ( plane=0; plane < nbplanes; plane++ )
 					{
@@ -326,13 +397,45 @@
 
 						for ( j=0; j<8; j++ )
 						{
-							if ( ( plane + j ) <= 7 ) ptr[j] |= (Uint8)( color & msk ) >> ( 7 - plane - j );
-							else 	                    ptr[j] |= (Uint8)( color & msk ) << ( plane + j - 7 );
+							if ( ( plane + j ) <= 7 ) colors[j] |= (Uint8)( color & msk ) >> ( 7 - plane - j );
+							else                      colors[j] |= (Uint8)( color & msk ) << ( plane + j - 7 );
 
 							msk >>= 1;
 						}
 					}
-					ptr += 8;
+
+					if ( is_ham )
+					{
+						for ( j=0; j<8; j++ )
+						{
+							color = colors[j] & ham_mask1;
+							switch ( colors[j] >> ham_shift1 )
+							{
+							case 0x00:
+								red   = colormap[3*color];
+								green = colormap[3*color+1];
+								blue  = colormap[3*color+2];
+								break;
+							case 0x01:
+								blue  = (  blue & ham_mask2 ) | ( color << ham_shift2 );
+								break;
+							case 0x02:
+								red   = (   red & ham_mask2 ) | ( color << ham_shift2 );
+								break;
+							case 0x03:
+								green = ( green & ham_mask2 ) | ( color << ham_shift2 );
+								break;
+							}
+							*ptr++ = red;
+							*ptr++ = green;
+							*ptr++ = blue;
+						}
+					}
+					else
+					{
+						memcpy( ptr, colors, 8 );
+						ptr += 8;
+					}
 				}
 			}
 			else
@@ -354,19 +457,9 @@
 								color24 = color24 | maskColor24;
 							maskColor24 = maskColor24<<1;
 						}
-						if ( SDL_BYTEORDER == SDL_LIL_ENDIAN )
-						{
-							*ptr++ = color24>>16;
-							*ptr++ = color24>>8;
-							*ptr++ = color24;
-						}
-						else
-						{
-							*ptr++ = color24;
-							*ptr++ = color24>>8;
-							*ptr++ = color24>>16;
-						}
-
+						*ptr++ = color24;
+						*ptr++ = color24>>8;
+						*ptr++ = color24>>16;
 						maskBit = maskBit>>1;
 					}
 				}