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Old 09-18-2008, 03:38 PM   #1
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Default Inaccurate pixel values using getPixel


I'm writing an applicaton which acts based on different colour values of pixels in a png image created in Microsoft paint. However, when I load the image into a BitmapData object, and then parse through the pixels reading their values using getPixel() or getPixel32() (I've tried both) I seem to be getting values which are slightly inaccurate compared to what was saved. For example a pure white pixel which should be read as 0xFFFFFFFF often comes out as 0xFFFFFFFE. The behaviour of the program is dependent on the exact value of the pixel so these errors usually break it.

Does anyone know why this is? Sometimes the problem is intermittent so seems fixed but actually isn't. Is it to do with the PNG format? I thouht png was lossless so should deliver accurate pixel values at all times?

Any help would be much appreciated.

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Old 09-18-2008, 03:44 PM   #2
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The only thing I can think that might cause the inconsistancy is if you capture the pixel for a object that has children not positioned at whole pixels themselves.. like if its coordinates are x:2.5 y:10.3, just a thought..
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Old 09-18-2008, 03:44 PM   #3
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are you loading the png from a file during runtime? Or did you import it into your library in Flash and then create an instance from the library?

If from the library, that would explain the differences. Flash, much to my chagrin, imports all images as JPG then converts them to be either JPG or PNG when compiling the SWF. For Flash, "lossless" actually means "lossless after we've already converted your image into a lossy format".

If from a file at runtime, it might possibly be from the PNG, but I wouldn't think so. You could try reloading the PNG into an image editor and check the pixels there again. I don't think Flash should be reporting invalid values for the colors if the image truely had different values.
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Old 09-18-2008, 04:51 PM   #4
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Thanks very much for your help. The pngs are being loaded from external files. However it seems someone else has been having the same problems:


I can only assume that Flash is applying some kind of compression within the bitmapdata class - to save on memory perhaps? Or perhaps it's doing some kind of weird anti-aliasing that we don't know about, which is altering the values.

Either way it looks like I'll have to find some alternative data storage...
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Old 09-18-2008, 05:06 PM   #5
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PNG is lossles, but, the editing tool may use palette with the fixed colors to generate a PNG. Almost always PNGs do not use the palette, but, still there's an option for it, I can imagine, that if you'll have a file with only few different colors, than the editing tool may automaticaly switch to use the palette (you'll have to check the input in another, more advanced editor to make sure the PNG was saved exactly as you wanted).
Also, if you transformed the image and used BitmapData.draw() on the transformed one, than there may be difference.
Oh, and by thee wee %) Have you saved your PNG as 8 bits per channel or 16 bits per channel?
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Last edited by wvxvw; 09-18-2008 at 05:16 PM.
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Old 03-23-2012, 08:31 AM   #6
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Default Window Ms Paint Error

I had this problem.
Turns out that when making a bitmap with MSPaint,
it only uses 0-254 for the pixel values.
I don't care what it's color sampler says, it lies.

I made a level editor that stored it's information in a PNG
and it also relied on exact colors. I ran into this problem
and it drove me crazy.
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