OpenGL ES - glReadPixels

问题:

I am taking a screenshot with glReadPixels to perform a "cross-over" effect between two images.

On the Marmalade SDK simulator, the screenshot is taken just fine and the "cross-over" effect works a treat:

However, this is how it looks on iOS and Android devices - corrupted: http://www.eikona.info/images/81269689420703803966.png

I always read the screen as RGBA 1 byte/channel, as the documentation says it's ALWAYS accepted.

Here is the code used to take the screenshot:

uint8* Gfx::ScreenshotBuffer(int& deviceWidth, int& deviceHeight, int& dataLength) {

    /// width/height
    deviceWidth = IwGxGetDeviceWidth();
    deviceHeight = IwGxGetDeviceHeight();
    int rowLength = deviceWidth * 4; /// data always returned by GL as RGBA, 1 byte/each

    dataLength = rowLength * deviceHeight;

    // set the target framebuffer to read
    glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
    glPixelStorei(GL_PACK_ALIGNMENT, 1);
    uint8* buffer = new uint8[dataLength];
    glReadPixels(0, 0, deviceWidth, deviceHeight, GL_RGBA, GL_UNSIGNED_BYTE, buffer);

    return buffer;
}

void Gfx::ScreenshotImage(CIwImage* img, uint8*& pbuffer) {

    int deviceWidth, deviceHeight, dataLength;

    pbuffer = ScreenshotBuffer(deviceWidth, deviceHeight, dataLength);
    img->SetFormat(CIwImage::ABGR_8888);
    img->SetWidth(deviceWidth);
    img->SetHeight(deviceHeight);
    img->SetBuffers(pbuffer, dataLength, 0, 0);
}

回答1:

That is a driver bug. Simple as that.

The driver got the pitch of the surface in the video memory wrong. You can clearly see this in the upper lines. Also the garbage you see at the lower part of the image is the memory where the driver thinks the image is stored but there is different data there. Textures / Vertex data maybe.

And sorry, I know of no way to fix that. You may have better luck with a different surface-format or by enabling/disabling multisampling.

回答2:

In the end, it was lack of memory. The "new uint8[dataLength];" never returned an existent pointer, thus the whole process went corrupted.

TomA, your idea of clearing the buffer actually helped me to solve the problem. Thanks.

回答3:

I don't know about android or the SDK you're using, but on IOS when I take a screenshot I have to make the buffer the size of the next POT texture, something like this:

int x = NextPot((int)screenSize.x*retina);
int y = NextPot((int)screenSize.y*retina);

void *buffer = malloc( x * y * 4 );

glReadPixels(0,0,x,y,GL_RGBA,GL_UNSIGNED_BYTE,buffer);

The function NextPot just gives me the next POT size, so if the screen size was 320x480, the x,y would be 512x512.

Maybe what your seeing is the wrap around of the buffer because it's expecting a bigger buffer size ?

Also this could be a reason for it to work in the simulator and not on the device, my graphics card doesn't have the POT size limitation and I get similar (weird looking) result.

回答4:

What I assume is happening is that you are trying to use glReadPixels on the window that is covered. If the view area is covered, then the result of glReadPixels is undefined.

See How do I use glDrawPixels() and glReadPixels()? and The Pixel Ownership Problem.

As said here :

The solution is to make an offscreen buffer (FBO) and render to the FBO.

Another option is to make sure the window is not covered when you use glReadPixels.

回答5:

I am getting screenshoot of my android game without any problems on android device using glReadPixels.

I am not sure yet what's the problem in your case, need more information. So lets start:

1. I would recommend you not to specify PixelStore format. I am worried about your alignment in 1 byte, do you really "use it"/"know what does it do"? It seems you get exactly what you specify - one extra byte(look at your image, there one extra pixel all the time!) instead of fully packed image. SO try to remove this:

   glPixelStorei(GL_UNPACK_ALIGNMENT, 1); glPixelStorei(GL_PACK_ALIGNMENT, 1);

2. I am not sure in C code, as I was working only in java, but this looks as possible point:

   // width/height deviceWidth = IwGxGetDeviceWidth(); deviceHeight = IwGxGetDeviceHeight();

Are you getting device size? You should use your OpenGL surface size, like this:

public void onSurfaceChanged(GL10 gl, int width, int height) {
int surfaceWidth = width;
int surfaceHeight = height;
}

1. What are you doing next with captured image? Are you aware that memory block you got from opengl is RGBA, but all not-opengl image operations expect ARGB? For example here in your code you expect alpha to be first bit, not last:

   img->SetFormat(CIwImage::ABGR_8888);

2. In case if 1, 2 and 3 did not help you might want to save the captured screen to phone sdcard to examine later. I have a program that converts opengl RGBA block to normal bitmap to examine on PC. I may share it with you.

回答6:

I don't have a solution for fixing glReadPixels. My suggestion is that you change your algorithm to avoid the need to read the data back from the screen.

Take a look at this page. These guys have done a page flip effect all in Flash. It's all in 2D, the illusion is achieved just with shadow gradients.

I think you can use a similar approach, but a little better in 3D. Basically you have to split the effect into three parts: the front facing top page (clouds), the bottom page (the girl) and the back side of the front page. You have to draw each part separately. You can easily draw the front facing top page and the bottom page together in the same screen, you just need to invoke the drawing code for each with a preset clipping region that is aligned with the split line where the top page bends. After you have to top and back page sections drawn, you can draw the gray back facing portion on top, also aligned to the split line.

With this approach the only thing you lose is a little bit of deformation where the clouds image starts to bend up, of course no deformation will occur with my method. Hopefully that will not diminish the effect, I think the shadows are way more important to give the depth effect and will hide this minor inconsistency.