black background in plane 2D texture

Hi.

I am implementing Vuforia on Android. I am using example ImageTargetsNative. I replace the teapot model with 2D plane image. I modified the native C++ rendering code as instructed here:

https://developer.vuforia.com/resources/dev-guide/textured-plane-targetEverything is OK. But now when I try to use my own png image (512x512), I always get a black background. My image has transparent background. Now the plane 2D image fits the target image, but left a black background.

Screenshot in the attachment, you can see the black background around the image of a skirt.

Can you please make any suggestion how to remove it? I wan the backbround to be transparent completely.

const Texture* const thisTexture = textures[textureIndex];

			QCAR::Matrix44F modelViewProjection;
			SampleUtils::translatePoseMatrix(0.0f, 0.0f, kObjectScale,
							 &modelViewMatrix.data[0]);
			SampleUtils::scalePoseMatrix(targetSize.data[0], targetSize.data[1], 1.0f,
						     &modelViewMatrix.data[0]);
			SampleUtils::multiplyMatrix(&projectionMatrix.data[0],
						    &modelViewMatrix.data[0] ,
						    &modelViewProjection.data[0]);
			glUseProgram(shaderProgramID);
			glVertexAttribPointer(vertexHandle, 3, GL_FLOAT, GL_FALSE, 0,
					      (const GLvoid*) &planeVertices[0]);
			glVertexAttribPointer(normalHandle, 3, GL_FLOAT, GL_FALSE, 0,
					      (const GLvoid*) &planeNormals[0]);
			glVertexAttribPointer(textureCoordHandle, 2, GL_FLOAT, GL_FALSE, 0,
					      (const GLvoid*) &planeTexcoords[0]);
			glEnableVertexAttribArray(vertexHandle);
			glEnableVertexAttribArray(normalHandle);
			glEnableVertexAttribArray(textureCoordHandle);
			glActiveTexture(GL_TEXTURE0);
			glBindTexture(GL_TEXTURE_2D, thisTexture->mTextureID);
			glUniformMatrix4fv(mvpMatrixHandle, 1, GL_FALSE,
					   (GLfloat*)&modelViewProjection.data[0] );
			glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT,
				       (const GLvoid*) &planeIndices[0]);

			glDisableVertexAttribArray(vertexHandle);
			glDisableVertexAttribArray(normalHandle);
			glDisableVertexAttribArray(textureCoordHandle);

Here is the rendering code from ImageTarget.cpp