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POandroid saving and loading PNG bitmaps differences
 

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  1. POandroid saving and loading PNG bitmaps differences
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    <p>I'm trying to solve my problem for a several days now and I couldn't figure it out. My application uses a Bitmap to process image's pixels in a similar way steganography does. After I modify Bitmap and get lets say ModifiedBitmap and check the pixel values everything is correct. Problems begins after I save the Bitmap as PNG with compress method. When I load ModifiedBitmap back to memory pixel values are changed. As I store the Bitmap in a PNG format with ARGB_8888 I assumed that the values will be the same after saving and loading the image in PNG format.</p> <p>For more detail I'm loading the bitmap with the following code:</p> <pre><code>BitmapFactory.Options opt = new BitmapFactory.Options(); opt.inDither = false; opt.inPreferredConfig = Bitmap.Config.ARGB_8888; BitmapFactory.decodeFile(picturePath, opt) </code></pre> <p>Whatever the Bitmap format is I process it to get the right Bitmap format for my purposes. My aim is to get Bitmap that has alpha and is in the ARGB_8888 config. I prepare new Bitmap with the following:</p> <pre><code>int width = this.imageToProcess.getWidth(); int height = this.imageToProcess.getHeight(); Bitmap dest = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888); for (int i = 0; i &lt; width; i++) { for (int j = 0; j &lt; height; j++) { int pixel = this.imageToProcess.getPixel(i, j); int newColor = 0; if (this.imageToProcess.hasAlpha()) { newColor = Color.argb(Color.alpha(pixel), Color.red(pixel), Color.green(pixel), Color.blue(pixel)); } else { newColor = Color.argb(255, Color.red(pixel), Color.green(pixel), Color.blue(pixel)); } dest.setPixel(i, j, newColor); } } </code></pre> <p>When I use such a prepare method I always get the right format of the Bitmap with the right parameters and configuration. So now, the Bitmap has config ARGB_8888 and has the alpha values. Lets say I've got the PreparedBitmap. I also checked the Bitmap properties and it is exactly ARGB_8888 and hasAlpha is true. So far now, it is all as it should be. Then I make some sort of pixel modifing on the PreparedBitmap and it is ready to be saved. I use the following code for saving the Bitmap as a PNG:</p> <pre><code>String path = Environment.getExternalStorageDirectory().toString(); OutputStream fOut = null; File file = new File(path, "image.png"); fOut = new FileOutputStream(file); modifiedBitmap.compress(Bitmap.CompressFormat.PNG, 100, fOut); fOut.flush(); fOut.close(); MediaStore.Images.Media.insertImage(getContentResolver(),file.getAbsolutePath(),file.getName(),file.getName()); </code></pre> <p>After that I have stored PNG image file. Then I want to load ModifiedBitmap again and process the Bitmap in inverse manner. My prerequisitie is that the ModifiedBitmap has exactly same pixel values as I modified it in the first step. I load the Bitmap through the same procedure:</p> <pre><code>BitmapFactory.Options opt = new BitmapFactory.Options(); opt.inDither = false; opt.inPreferredConfig = Bitmap.Config.ARGB_8888; BitmapFactory.decodeFile(picturePath, opt) </code></pre> <p>But he loaded ModifiedBitmap when it was saved before has different pixel values. Also it hasn't alpha values (hasAlpha is false) but the config is the same ARGB_8888. Does anybody know why is that? It is ARGB_8888 Bitmap so there shouldn't be a differences in pixel values. Does android compress PNG Bitmap somehow? How should I save the PNG images to prevent from pixel values changes?</p> <p>If the ModifiedBitmap pixel values is in ARGB_8888 (128, 128, 128, 128) and I save it through Bitmap.compress method I want to get the same values with alpha after loading it again. I'm out of all ideas how to do that.</p>
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    1. COCould it be because of premultiplication of alpha during save or reload?
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