imageData.data.indexOf() doesn't work

[troido]

Hello,I want to make a script to check if there is anything drawn on a canvas.I tried to use this script:

[/font][font=arial,helvetica,sans-serif]var ctx = document.getElementById("canvas").getContext("2d");[/font][font=arial,helvetica,sans-serif]/* Some other code, including some drawing code[/font][font=arial,helvetica,sans-serif]All drawings are black, with alpha value 255 */[/font] [font=arial,helvetica,sans-serif]var a = ctx.[color=#000000][size=1]getImageData(0,0,room.width,room.height)[/size][/color].data.indexOf(255); // to see if anything is drawn[/font][font=arial,helvetica,sans-serif]

But for some reason it didn't work.I have also tested some other array methods, but they didn't work either on the imageData.dataIs there some way to make this work?Using a for loop instead would probably be to slow. Michiel

Edited December 2, 2012 by troido

[JamesB]

are you sure imageData is a property of the context object? you might need getImageData() instead.i'm not sure how to use indexOf on an ArrayBuffer, but the loop might be worth a try:

var canvas = document.getElementById("canvas");var ctx = canvas.getContext("2d");var imageData = ctx.getImageData(0, 0, canvas.width, canvas.height); var data = imageData.data;var dataLength = data.length; var alphaFound = false;for (var i=0; i<dataLength; i+=4){if (data[i] == 255){alphaFound = true;break;}}

[troido]

Yes, i meant getImageData(...).In the code I use I had this, but i made a mistake in the post.I corrected the post now. The loop will probably work (if var i starts as 3 instead of 0) but i wanted to know if there is also an solution that works a little faster.