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Dabbling around with OGLES in Android. I want to create some triangles and translate them. Straightforward enough.

I look at the examples and see that FloatBuffers are recommended as the storage construct for polygon coordinates using GLES in Android, because they are on native code steroids. Good stuff! Explanation here.

Why FloatBuffer instead of float[]?

Now what's confusing to me is the Matrix class in Android operates almost exclusively on java float arrays. So on one hand I'm being told to use FloatBuffers for speed, but on the other hand every transformation I want to do now has to look like this?

float[] myJavaFloatArray = new float[polylength];
myFloatBuffer.get(myJavaFloatArray);
Matrix.translateM(myJavaFloatArray,0,-1.0f,0.0f,0.0f);
myFloatBuffer.put(myJavaFloatArray);

Is this really the best practice? Is the assumption just that my ratio of translates to draws will be low enough that this expense is worth it? What if I'm doing lots of translates, one on every object per frame, is that an argument for using java float arrays throughout?

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Monolithguy
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1 Answers1

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First of all, isn't there a method array() for FloatBuffer that returns float[] value pointing directly to the buffer data? You should use that in translate method so you don't copy the data around the memory, this should turn your 4 lines of code into a single line:

Matrix.translateM(myFloatBuffer.array(),0,-1.0f,0.0f,0.0f);

Second it is not best practice to transform your vertex data on the CPU, you have a vertex shader for that. Use matrices and push them to the GPU so you can benefit from it. Also if you transform those data the way you do you kind of corrupt them: If the float buffer contained vertex array for some object you want to draw in several location you would have to either translate it back to 0 before translating it to a new location for each draw call or have multiple instances of the vertex data.

Any way, although it can be very handy to be able to transform the array buffer you should avoid using it per frame if possible. Do it in loading time or in background when preparing some element. As for per frame rather try to use vertex shader for that. In case you are dealing with ES1 and you have no vertex shader there are still matrix operations that work in the same way (translate, rotate, scale, set, push, pop...)

Matic Oblak
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  • Thanks for the answer. The array() method is supposed to do exactly what you said but on my device it threw errors. Turns out there is no guarantee that method is implemented on a target. That said I'll try the shader approach first thing when I get home! – Monolithguy Jun 21 '13 at 22:34