问题
I am very new to OpenGL and I'm doing a very simple program to check if I can pick objects rendered on the screen, drawing each one with a unique color. So far, I render 2 flat squares and one triangle.
The problem is that when the user touches the polygons, I get a color different from black, but the color index is the same for the three objects (when it should be different).
Here is my code:
The onDrawFrame method (within my Renderer). Here is where I render the polygons with an unique color when the user is touching the screen.
public void onDrawFrame(GL10 gl) {
// clear Screen and Depth Buffer
gl.glClear(GL10.GL_COLOR_BUFFER_BIT | GL10.GL_DEPTH_BUFFER_BIT);
// Reset the Modelview Matrix
gl.glLoadIdentity();
if (mGLView.isPickingEnabled()) {
gl.glDisable(GL10.GL_TEXTURE_2D);
gl.glDisable(GL10.GL_LIGHTING);
gl.glDisable(GL10.GL_FOG);
// draw colored objects and picking pixel color
pickingDraw(gl);
mGLView.mPickingY = (int)(mHeight - mGLView.mPickingY);
gl.glReadPixels((int) mGLView.mPickingX,
(int) mGLView.mPickingY, 1, 1,
GL10.GL_RGBA,
GL10.GL_UNSIGNED_BYTE,
mGLView.m_PickingPixelBuffer);
Log.i("RA", "RGBA COLORS: " +
(int) mGLView.m_PickingPixelBuffer.get(0) + " , " +
(int) mGLView.m_PickingPixelBuffer.get(1) + " , " +
(int) mGLView.m_PickingPixelBuffer.get(2) + " , " +
(int) mGLView.m_PickingPixelBuffer.get(3) );
mGLView.disablePicking();
// Clear BACK BUFFER
gl.glClear(GL10.GL_COLOR_BUFFER_BIT | GL10.GL_DEPTH_BUFFER_BIT);
}
gl.glLoadIdentity();
// Drawing
gl.glTranslatef(0.0f, 0.0f, -25.0f); // move 5 units INTO the screen
// is the same as moving the camera 5 units away
square.draw(gl); // Draw the triangle
gl.glTranslatef(0.0f, -5.0f, 0.0f);
square.draw(gl);
gl.glTranslatef(4.0f, 10.0f, 0.0f);
triangle.draw(gl);
}
pickingDraw method is the following (within my Renderer too):
private void pickingDraw(GL10 gl) {
// Drawing
gl.glTranslatef(0.0f, 0.0f, -25.0f); // move 5 units INTO the screen
gl.glColor4f(128, 0, 0, 1);
square.draw(gl); // Draw the triangle
gl.glTranslatef(0.0f, -5.0f, 0.0f);
gl.glColor4f(129, 0, 0, 1);
square.draw(gl);
gl.glTranslatef(4.0f, 10.0f, 0.0f);
gl.glColor4f(130, 0, 0, 1);
triangle.draw(gl);
}
On the other hand, I inherited GLSurfaceView so I can handle user touches. Within this class, I use this method:
public boolean onTouchEvent(MotionEvent e) {
float _iX = e.getX();
float _iY = e.getY();
int iIndex = -1;
int iRed;
m_PickingPixelBuffer = ByteBuffer.allocateDirect(4);
m_PickingPixelBuffer.order(ByteOrder.nativeOrder());
// setting globals for Renderer
mPickingX = _iX;
mPickingY = _iY;
mPickingEnabled = true;
requestRender();
// wait for picking process (renderer)
while(mPickingEnabled);
iRed = m_PickingPixelBuffer.get(0);
if(iRed < 0)
{
iRed += 256;
}
Log.i("RA", "Coordinates: (" + mPickingX + ", " + mPickingY + ")");
Log.i("RA", "Red Color " + iRed);
return true;
}
It seems that it detects the squares and the triangle when I touch them, but I get the same color index for the three of them. Plus, the three objects have the same exact color (I even drew each one with a value of red, green and blue respectively and they had the same color, the color of the last one that was drawn).
Finally, this is the output I get when I touch an square, then an empty space, and another square:
01-19 01:34:23.345: INFO/RA(11566): RGBA COLORS: -1 , 0 , 0 , -1
01-19 01:34:23.355: INFO/RA(11566): Coordinates: (247.81186, 391.0)
01-19 01:34:23.355: INFO/RA(11566): Red Color 255
01-19 01:34:23.385: INFO/RA(11566): RGBA COLORS: -1 , 0 , 0 , -1
01-19 01:34:23.385: INFO/RA(11566): Coordinates: (248.52428, 390.0)
01-19 01:34:23.385: INFO/RA(11566): Red Color 255
01-19 01:34:23.405: INFO/RA(11566): RGBA COLORS: -1 , 0 , 0 , -1
01-19 01:34:23.405: INFO/RA(11566): Coordinates: (248.52428, 390.0)
01-19 01:34:23.405: INFO/RA(11566): Red Color 255
01-19 01:34:24.305: INFO/RA(11566): RGBA COLORS: 0 , 0 , 0 , -1
01-19 01:34:24.315: INFO/RA(11566): Coordinates: (119.571884, 577.0)
01-19 01:34:24.315: INFO/RA(11566): Red Color 0
01-19 01:34:24.335: INFO/RA(11566): RGBA COLORS: 0 , 0 , 0 , -1
01-19 01:34:24.335: INFO/RA(11566): Coordinates: (119.33441, 573.0)
01-19 01:34:24.335: INFO/RA(11566): Red Color 0
01-19 01:34:24.355: INFO/RA(11566): RGBA COLORS: 0 , 0 , 0 , -1
01-19 01:34:24.355: INFO/RA(11566): Coordinates: (119.33441, 573.0)
01-19 01:34:24.355: INFO/RA(11566): Red Color 0
01-19 01:34:25.195: INFO/RA(11566): RGBA COLORS: -1 , 0 , 0 , -1
01-19 01:34:25.195: INFO/RA(11566): Coordinates: (259.80466, 395.0)
01-19 01:34:25.195: INFO/RA(11566): Red Color 255
01-19 01:34:25.215: INFO/RA(11566): RGBA COLORS: -1 , 0 , 0 , -1
01-19 01:34:25.215: INFO/RA(11566): Coordinates: (259.44843, 398.0)
01-19 01:34:25.215: INFO/RA(11566): Red Color 255
01-19 01:34:25.245: INFO/RA(11566): RGBA COLORS: -1 , 0 , 0 , -1
01-19 01:34:25.245: INFO/RA(11566): Coordinates: (259.80466, 395.0)
01-19 01:34:25.245: INFO/RA(11566): Red Color 255
I don't know why the red values are the same for each object.
Any help would be appreciated. Thanks.
回答1:
gl.glColor4f(red, green, blue, alpha); the type of the four parameters are float and the value are supposed to be 0 to 1, which match the common RGB range 0 to 255 so if you set the R value 128, 129 and 130, in fact , there is no difference between them. They are all 255 for common RGB range.
回答2:
Long long ago, in a galaxy far away...
Ok, this question is history, but if someone ever should have the same problem, it's worth checking whether DITHERING is turned off.
I had a similar issue once, and that was the issue for me.
来源：https://stackoverflow.com/questions/7488197/picking-objects-in-android-opengl-es-using-colors-does-not-retrieve-correct-colo