is there a better way to double/quad up a bit pattern?

Post #347,562 by SpiceWare 9/9/11 7:20:15 PM 9/10/11 10:50:46 PM Reply	is there a better way to double/quad up a bit pattern? For my latest Atari 2600 game I'm having to use software collision detection. I'm about to add support for wide sprites (the Atari can 2x and 4x horizontally) for some new objects and need to revise the collision routines to handle that. Basically if a bit pattern is _X_XX_X_ I need to test the collision against __XX__XXXX__XX__. I've written this up (but haven't tested it yet) and thought somebody here might have a better way to do it. int DoubleBits(int bits) { int i; int mask; int result; mask = 0x80; result = 0; for(i=0;i<8;i++) { result = (result << 2) + ((bits & mask) ? 3 : 0); mask >>= 1; } return result; } To make that a QuadBits for the 4x sprites, I'd just change the 1 line to result = (result << 4) + ((bits & mask) ? 15 : 0); ~~Hmm, I know there's a way to post formatted code...~~ Edited by SpiceWare Sept. 10, 2011, 10:50:46 PM EDT Expand All History
Post #347,572 by malraux 9/9/11 10:46:30 PM Reply	Try these http://graphics.stan...leaveTableObvious If you have space but not time, the table lookup method is probably best. Regards, -scott Welcome to Rivendell, Mr. Anderson.
Post #347,573 by Another Scott 9/9/11 11:01:12 PM Reply	Good stuff. Any (known) relation?
Post #347,579 by malraux 9/10/11 10:53:53 AM Reply	Hah, I didn't notice that the first time Regards, -scott Welcome to Rivendell, Mr. Anderson.
Post #347,574 by SpiceWare 9/9/11 11:19:45 PM Reply	thanks looks like I'm doing a variation of the "obvious way", though mine is interleaving the value with itself. table lookup would probably be best - think I'll do it with a 16 byte table and double up each nybble as a 512 byte table would take up too much room.
Post #347,627 by drook 9/11/11 9:38:42 AM Reply	I like the page title I read it as, "A collection of code for by and for bit twiddling hacks." -- Drew
Post #347,617 by SpiceWare 9/10/11 10:51:46 PM Reply	my table driven routine const unsigned char double_nybble[] = { 0x00, 0x03, 0x0C, 0x0F, 0x30, 0x33, 0x3C, 0x3F, 0xC0, 0xC3, 0xCC, 0xCF, 0xF0, 0xF3, 0xFC, 0xFF}; int DoubleBits(int bits) { return (double_nybble[(bits & 0xf0) >> 4] << 8) + double_nybble[bits & 0x0f]; }
Post #347,618 by malraux 9/10/11 11:07:03 PM Reply	Nice, looks like it should be quick Regards, -scott Welcome to Rivendell, Mr. Anderson.
Post #347,621 by pwhysall 9/11/11 3:52:32 AM Reply	Re: my table driven routine Haven't seen the word "nybble" since my student days.

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