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How to get OpenGL screenshot ? Useful programing hint.

Tutorials concerning the OpenGL® ES cross-platform API for full-function 2D and 3D graphics on the Google-Android platform.
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How to get OpenGL screenshot ? Useful programing hint.

Postby eugenk » Sun Feb 10, 2008 10:56 am

How to get OpenGL screenshot ? Useful programing hint.


Screenshot is not only addictive forum topic appearance element. It can be very important in graphics algorithm debugging. Indeed, it can be opened in gimp or photoshop, and examined in detail. So, method, how to get screenshot from exactly defined screen area, is important programmers tool. Let to do it.

At first, save picture as bitmap

Syntax: [ Download ] [ Hide ]
Using java Syntax Highlighting
  1.      public static Bitmap SavePixels(int x, int y, int w, int h, GL10 gl)
  2.     {  
  3.          int b[]=new int[w*h];
  4.          int bt[]=new int[w*h];
  5.          IntBuffer ib=IntBuffer.wrap(b);
  6.          ib.position(0);
  7.          gl.glReadPixels(x, y, w, h, GL10.GL_RGBA, GL10.GL_UNSIGNED_BYTE, ib);
  8.          for(int i=0; i<h; i++)
  9.          {//remember, that OpenGL bitmap is incompatible with Android bitmap
  10.           //and so, some correction need.        
  11.               for(int j=0; j<w; j++)
  12.               {
  13.                    int pix=b[i*w+j];
  14.                    int pb=(pix>>16)&0xff;
  15.                    int pr=(pix<<16)&0x00ff0000;
  16.                    int pix1=(pix&0xff00ff00) | pr | pb;
  17.                    bt[(h-i-1)*w+j]=pix1;
  18.               }
  19.          }                  
  20.          Bitmap sb=Bitmap.createBitmap(bt, w, h, true);
  21.          return sb;
  22.     }
  23.  
Parsed in 0.033 seconds, using GeSHi 1.0.8.4


Then save Android picture as png-file.
Syntax: [ Download ] [ Hide ]
Using java Syntax Highlighting
  1.         public static void SavePNG(int x, int y, int w, int h, String name, GL10 gl)
  2.         {
  3.                 Bitmap bmp=SavePixels(x,y,w,h,gl);
  4.                 try
  5.                 {
  6.                         FileOutputStream fos=new FileOutputStream("/sdcard/my_app/"+name);
  7.                         bmp.compress(CompressFormat.PNG, 100, fos);
  8.                         try
  9.                         {
  10.                                 fos.flush();
  11.                         }
  12.                         catch (IOException e)
  13.                         {
  14.                                 // TODO Auto-generated catch block
  15.                                 e.printStackTrace();
  16.                         }
  17.                         try
  18.                         {
  19.                                 fos.close();
  20.                         }
  21.                         catch (IOException e)
  22.                         {
  23.                                 // TODO Auto-generated catch block
  24.                                 e.printStackTrace();
  25.                         }
  26.                        
  27.                 }
  28.                 catch (FileNotFoundException e)
  29.                 {
  30.                         // TODO Auto-generated catch block
  31.                         e.printStackTrace();
  32.                 }              
  33.         }
  34. }
  35.  
Parsed in 0.032 seconds, using GeSHi 1.0.8.4


Finally use it all.
Syntax: [ Download ] [ Hide ]
Using java Syntax Highlighting
  1. Util.SavePNG(0, 0, Config.SCREEN_W, Config.SCREEN_H, "testpicture.png", gl);
  2.  
Parsed in 0.033 seconds, using GeSHi 1.0.8.4


Now, look at result :)
Attachments
testpicture.png
testpicture.png (37.08 KiB) Viewed 16765 times
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Postby benny! » Wed Feb 13, 2008 9:24 pm

Very useful. I will test it. Thank you very much.

Keep OpenGL|ES tricks coming ;-)
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Postby eugenk » Sat Feb 16, 2008 1:36 pm

benny!, thank ! I promise that my OpenGL topics will be continued and other topics about Android programming tips and tricks will be begin, when I'll have some free time. Now I have not it :(

Guys ! About trick, described it this topic. IT NOT WORKS. Matter is in glReadPixels() method. Current SDK version has a BUG in glReadPixels(). Now this method not react to y parameter, and already assumes that y=0. So to fix it, SavePixels() method must be rewritten as:
Syntax: [ Download ] [ Hide ]
Using java Syntax Highlighting
  1. //SavePixels patch. It is necessary because glReadPizels has a BUG
  2. //it not process parameter y, and sets it to zero      
  3.     public static Bitmap SavePixels(int x, int y, int w, int h, GL10 gl)
  4.     {  
  5.          int b[]=new int[w*(y+h)];
  6.          int bt[]=new int[w*h];
  7.          IntBuffer ib=IntBuffer.wrap(b);
  8.          ib.position(0);
  9.          gl.glReadPixels(x, 0, w, y+h, GL10.GL_RGBA, GL10.GL_UNSIGNED_BYTE, ib);
  10.          
  11.          for(int i=0, k=0; i<h; i++, k++)
  12.          {//remember, that OpenGL bitmap is incompatible with Android bitmap
  13.           //and so, some correction need.        
  14.               for(int j=0; j<w; j++)
  15.               {
  16.                    int pix=b[i*w+j];
  17.                    int pb=(pix>>16)&0xff;
  18.                    int pr=(pix<<16)&0x00ff0000;
  19.                    int pix1=(pix&0xff00ff00) | pr | pb;
  20.                    bt[(h-k-1)*w+j]=pix1;
  21.               }
  22.          }
  23.  
  24.          
  25.          Bitmap sb=Bitmap.createBitmap(bt, w, h, true);
  26.          return sb;
  27.     }
  28.  
  29.  
  30.  
Parsed in 0.038 seconds, using GeSHi 1.0.8.4


Sorry for code, that was not be checked in details.
Regards,
Eugene.
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Re: How to get OpenGL screenshot ? Useful programing hint.

Postby Gentoo » Mon Jan 31, 2011 10:43 pm

Hello all!
I'm new to android development. I've wrote my first android app using AndEngine & andeinginephysicsbox2dextension, so I have a background and some physics objects at the scene on a different layers. Next I need to get an openGL screenshot (bitmap with no need to save it into the file), but there is a problem... Help me please, how can I get an instance of GL10 to pass it into the method SavePixels(int x, int y, int w, int h, GL10 gl)? Any help will be great! Thanks.

Best Regards,
Vladimir.
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Re: How to get OpenGL screenshot ? Useful programing hint.

Postby Sakthi Sai » Sat Dec 03, 2011 2:09 pm

hi all,
Facing same problem as Gentoo.. Please help me.. I am using CCLayer in my application, need to take screen shot of it.. Tried to get GL10 instance in draw().. but the stored image is a black screen.. Any help is much appreciated..
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Re: How to get OpenGL screenshot ? Useful programing hint.

Postby Sakthi Sai » Fri Dec 09, 2011 7:31 am

Why is that? I am getting only black screen image. Please help me.. I have been stuck on this for many days.. I dont know where I am wrong.. I have implemented the same code as above.. Need help.. Please help me friends..
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Re: How to get OpenGL screenshot ? Useful programing hint.

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Why is that? I am getting only black screen image. Please help me.. I have been stuck on this for many days.. I dont know where I am wrong.. I have implemented the same code as above.. Need help.. Please help me friends..

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Re: How to get OpenGL screenshot ? Useful programing hint.

Postby sstteevvee » Thu Apr 19, 2012 1:39 pm

Thanks for the code.. It was very useful.

I've made an improvement to the part that converts from OpenGL bitmap format, to the Android bitmap format. Rather than manipulating the bytes directly, we can do it with a couple of API calls.

On my phone, this gives a speedup of over 10 times! What used to take several seconds is now well under half a second. Also, I think its memory use is about 30% less.

Syntax: [ Download ] [ Hide ]
Using java Syntax Highlighting
  1. public Bitmap SavePixels(GL10 gl)
  2.         {
  3.             int b[] = new int[Width * Height];
  4.             IntBuffer ib = IntBuffer.wrap(b);
  5.             ib.position(0);
  6.             gl.glReadPixels(0, 0, Width, Height, GL10.GL_RGBA, GL10.GL_UNSIGNED_BYTE, ib);
  7.  
  8.             // The bytes within the ints are in the wrong order for android, but convert into a
  9.             // bitmap anyway. They're also bottom-to-top rather than top-to-bottom. We'll fix
  10.             // this up soon using some fast API calls.
  11.             Bitmap glbitmap = Bitmap.createBitmap(b, Width, Height, Bitmap.Config.ARGB_4444);
  12.             ib = null; // we're done with ib
  13.             b = null; // we're done with b, so allow the memory to be freed
  14.  
  15.             // To swap the color channels, we'll use a ColorMatrix/ColorMatrixFilter. From the Android docs:
  16.             //
  17.             // This is a 5x4 matrix: [ a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t ]
  18.             // When applied to a color [r, g, b, a] the resulting color is computed as (after clamping):
  19.             //
  20.             // R' = a*R + b*G + c*B + d*A + e;
  21.             // G' = f*R + g*G + h*B + i*A + j;
  22.             // B' = k*R + l*G + m*B + n*A + o;
  23.             // A' = p*R + q*G + r*B + s*A + t;
  24.             //
  25.             // We want to swap R and B, so the coefficients will be:
  26.             // R' = B => 0,0,1,0,0
  27.             // G' = G => 0,1,0,0,0
  28.             // B' = R => 1,0,0,0,0
  29.             // A' = A => 0,0,0,1,0
  30.  
  31.             final float[] cmVals = { 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 1, 0 };
  32.  
  33.             Paint paint = new Paint();
  34.             paint.setColorFilter(new ColorMatrixColorFilter(new ColorMatrix(cmVals))); // our R<->B swapping paint
  35.  
  36.             Bitmap bitmap = Bitmap.createBitmap(Width, Height, Config.ARGB_4444); // the bitmap we're going to draw onto
  37.             Canvas canvas = new Canvas(bitmap); // we draw to the bitmap through a canvas
  38.             canvas.drawBitmap(glbitmap, 0, 0, paint); // draw the opengl bitmap onto the canvas, using the color swapping paint
  39.             glbitmap = null; // we're done with glbitmap, let go of its memory
  40.  
  41.             // the image is still upside-down, so vertically flip it
  42.             Matrix matrix = new Matrix();
  43.             matrix.preScale(1.0f, -1.0f); // scaling: x = x, y = -y, i.e. vertically flip
  44.             return Bitmap.createBitmap(bitmap, 0, 0, bitmap.getWidth(), bitmap.getHeight(), matrix, true); // new bitmap, using the flipping matrix
  45.         }
  46.  
Parsed in 0.043 seconds, using GeSHi 1.0.8.4
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