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POImage transparency darkened when saved using OpenCv
 

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  1. POImage transparency darkened when saved using OpenCv
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    <p>I created a drawing application where I allow the user to draw and save the image to later reload to continue drawing. Essentially, I'm passing the drawing as a bitmap to the JNI layer to be saved and the same to load a previous drawing.</p> <p>I'm using OpenCv to write and read to png file.</p> <p>I'm noticing something weird in terms of the transparencies of the image. It almost seems as the transparency is being calculated against a black color on OpenCv? Take a look a the images attached, the contain lines that have transparencies.</p> <p><strong>Correct transparency by passing int array to native code, no color conversion needed:</strong> <img src="https://i.stack.imgur.com/pulHh.jpg" alt="enter image description here"></p> <p><strong>Darkened transparency by passing Bitmap object to native code, color conversion needed:</strong> <img src="https://i.stack.imgur.com/MsfuE.jpg" alt="enter image description here"></p> <p>What could potentially be happening?</p> <p><strong>Saving image using native Bitmap get pixel methods:</strong> </p> <pre><code>if ((error = AndroidBitmap_getInfo(pEnv, jbitmap, &amp;info)) &lt; 0) { LOGE("AndroidBitmap_getInfo() failed! error:%d",error); } if (0 == error) { if ((error = AndroidBitmap_lockPixels(pEnv, jbitmap, &amp;pixels)) &lt; 0) { LOGE("AndroidBitmap_lockPixels() failed ! error=%d", error); } } if (0 == error) { if (info.format == ANDROID_BITMAP_FORMAT_RGBA_8888) { LOGI("ANDROID_BITMAP_FORMAT_RGBA_8888"); } else { LOGI("ANDROID_BITMAP_FORMAT %d",info.format); } Mat bgra(info.height, info.width, CV_8UC4, pixels); Mat image; //bgra.copyTo(image); // fix pixel order RGBA -&gt; BGRA cvtColor(bgra, image, COLOR_RGBA2BGRA); vector&lt;int&gt; compression_params; compression_params.push_back(CV_IMWRITE_PNG_COMPRESSION); compression_params.push_back(3); // save image if (!imwrite(filePath, image, compression_params)) { LOGE("saveImage() -&gt; Error saving image!"); error = -7; } // release locked pixels AndroidBitmap_unlockPixels(pEnv, jbitmap); } </code></pre> <p><strong>Saving image using native int pixel array methods:</strong> </p> <pre><code>JNIEXPORT void JNICALL Java_com_vblast_smasher_Smasher_saveImageRaw (JNIEnv *pEnv, jobject obj, jstring jFilePath, jintArray jbgra, jint options, jint compression) { jint* _bgra = pEnv-&gt;GetIntArrayElements(jbgra, 0); const char *filePath = pEnv-&gt;GetStringUTFChars(jFilePath, 0); if (NULL != filePath) { Mat image; Mat bgra(outputHeight, outputWidth, CV_8UC4, (unsigned char *)_bgra); bgra.copyTo(image); if (0 == options) { // replace existing cache value mpCache-&gt;insert(filePath, image); } vector&lt;int&gt; compression_params; compression_params.push_back(CV_IMWRITE_PNG_COMPRESSION); compression_params.push_back(compression); // save image if (!imwrite(filePath, image)) { LOGE("saveImage() -&gt; Error saving image!"); } } pEnv-&gt;ReleaseIntArrayElements(jbgra, _bgra, 0); pEnv-&gt;ReleaseStringUTFChars(jFilePath, filePath); } </code></pre> <p><strong>Update 05/25/12:</strong><br> After a little more research I'm finding out that this issue does not happen if I get the int array of pixels from the bitmap and pass that directly to the JNI as opposed to what I do currently which is pass the entire Bitmap to the JNI layer then get the pixels and use <code>cvtColor</code> to convert pixels properly. Am I using the right pixel conversion?</p>
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    1. COWhat is the value of `compression` ?
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    2. COThe value I've tried was 0, 3 and 9. They all produce the same results.
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    3. COWhat version is this, and where did you downloaded it from?
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