javascript - Collision detection on chip8 emulator -

i made chip8 emulator, ran problems detecting collision when drawing. chip8 draws onto screen xor'ing individual pixels onto screen, , setting flag if pixel turned off. code follows:

_dxyn: function(x, y, n) {     console.log("_dxyn");      (var = 0; < n; i++) {         (var j = 0; j < 8; j++) {             var bitbefore = graphics[(i + cpu.registers.v[y]) * 64 + j + cpu.registers.v[x]];             var bitafter = bitbefore ^ cpu.getbits(memory[cpu.registers.i + i])[j];             graphics[(i + cpu.registers.v[y]) * 64 + j + cpu.registers.v[x]] = bitafter;              if ((bitbefore != bitafter) && bitbefore == 1)                 cpu.registers.v[0xf] = 0x1;             else                 cpu.registers.v[0xf] = 0x0;         }     } } 

graphics 1-dimensional array of ints, each corresponding pixel on screen. pixel on if corresponding integer in array 1, , off it 0. cpu object contains methods, including one, registers. x opcode parameter register contains x coordinate of sprite draw, , y opcode parameter register y coordinate. i register location in memory read from. cpu.getbits function follows:

getbits: function(opcode) {     var bits = [];     (var = 0; < 8; i++) {         bits.push((opcode >> i) & 1)     }     return bits.reverse(); } 

the complete code can found on github here: https://github.com/ichub/chip8 in chip8cpu.js implementation.

i found website : http://www.multigesture.net/articles/how-to-write-an-emulator-chip-8-interpreter explains in detail how write chip-8 interpreter. , wikipedia aticle on chip-8 think routine should following:

_dxyn: function(x, y, n) {     console.log("_dxyn");      cpu.registers.v[0xf] = 0x0;     (var = 0; < n; i++) { // rows of sprite         (var j = 0; j < 8; j++) {             var bitbefore = graphics[(i + cpu.registers.v[y]) * 64 + j +                              cpu.registers.v[x]];             var bitafter = bitbefore ^ cpu.getbits(memory[cpu.registers.i + i])[j];             graphics[(i + cpu.registers.v[y]) * 64 + j + cpu.registers.v[x]] = bitafter;              if ((bitbefore != bitafter) && bitbefore == 1)                 cpu.registers.v[0xf] = 0x1;         }     } } 

in routine vf cleared @ each pixel if there no collision value depends solely on last pixel drawn. should clear vf before starting draw, , set when there collision.