Cedric Fung

Co-founder of Shou

Stream the future to the present

Faster Alternatives to glReadPixels and glTexImage2D in OpenGL ES

Nov 05, 2013

In the development of Shou, I've been using GLSL with NEON to manipulate image rotation, scaling and color conversion, before send them to video encoder.

So I need a very efficient way to transfer pixels between OpenGL and memory space. The glTexImage2D and glReadPixels performance are very unacceptable, especially for some specific vendors, e.g. Samsung Galaxy devices with Exynos chip.

Compared to glTex(Sub)Image2D, the glReadPixels is the real bottleneck, which blocks all OpenGL pipeline and results in about 100ms delay for a standard 720P frame read back.

Here I will share two standard OpenGL approaches to achieve really fast pixels pack, which should be available on all OpenGL implementations. Only glReadPixels will be discussed, as the glTexImage2D should have the same usage.

Pixel Buffer Object

PBO is not introduced until OpenGL ES 3.0, which is available since Android 4.3. The pixels pack operation will be reduced to about 5ms using PBO.

PBO is created just like any other buffer objects:

glGenBuffers(1, &pbo_id);
glBindBuffer(GL_PIXEL_PACK_BUFFER, pbo_id);
glBufferData(GL_PIXEL_PACK_BUFFER, pbo_size, 0, GL_DYNAMIC_READ);
glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);

According to the reference of glReadPixels:

If a non-zero named buffer object is bound to the GL_PIXEL_PACK_BUFFER target (see glBindBuffer) while a block of pixels is requested, data is treated as a byte offset into the buffer object's data store rather than a pointer to client memory.

When we need to read pixels from an FBO:

glReadBuffer(GL_COLOR_ATTACHMENT0);
glBindBuffer(GL_PIXEL_PACK_BUFFER, pbo_id);
glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE, 0);
GLubyte *ptr = glMapBufferRange(GL_PIXEL_PACK_BUFFER, 0, pbo_size, GL_MAP_READ_BIT);
memcpy(pixels, ptr, pbo_size);
glUnmapBuffer(GL_PIXEL_PACK_BUFFER);
glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);

In a real project, we may consider using double or triple PBOs to improve the performance.

EGLImage

EGL_KHR_image_base is a completed EGL extension, which achieves the same performance as PBO, but only require OpenGL-ES 1.1 or 2.0.

The function to create an EGLImageKHR is

EGLImageKHR eglCreateImageKHR(EGLDisplay dpy,
                              EGLContext ctx,
                              EGLenum target,
                              EGLClientBuffer buffer,
                              const EGLint *attrib_list)

The Android EGL implementation frameworks/native/opengl/libagl/egl.cpp implies that the EGLDisplay should be a valid display, the EGLClientBuffer type should be ANativeWindowBuffer, the EGLContext can only be EGL_NO_CONTEXT, and the target can only be EGL_NATIVE_BUFFER_ANDROID.

All the other parameters are obvious, except for the ANativeWindowBuffer, which is defined in system/core/include/system/window.h.

To allocate an ANativeWindowBuffer, Android has a simple wrapper called GraphicBuffer, defined in frameworks/native/include/ui/GraphicBuffer.h.

GraphicBuffer *window = new GraphicBuffer(width, height, PIXEL_FORMAT_RGBA_8888, GraphicBuffer::USAGE_SW_READ_OFTEN | GraphicBuffer::USAGE_HW_TEXTURE);

struct ANativeWindowBuffer *buffer = window->getNativeBuffer();
EGLImageKHR *image = eglCreateImageKHR(eglGetCurrentDisplay(), EGL_NO_CONTEXT, EGL_NATIVE_BUFFER_ANDROID, *attribs);

Then anytime we want to read pixels from an FBO, we should use one of the two methods below:

void EGLImageTargetTexture2DOES(enum target, eglImageOES image)

void EGLImageTargetRenderbufferStorageOES(enum target, eglImageOES image)

These two methods will establishes all the properties of the target GL_TEXTURE_2D or GL_RENDERBUFFER.

uint8_t *ptr;
glBindTexture(GL_TEXTURE_2D, texture_id);
glEGLImageTargetTexture2DOES(GL_TEXTURE_2D, image);

window->lock(GraphicBuffer::USAGE_SW_READ_OFTEN, &ptr);
memcpy(pixels, ptr, width * height * 4);
window->unlock();

References

  1. GL_PIXEL_PACK_BUFFER http://www.khronos.org/opengles/sdk/docs/man3/xhtml/glMapBufferRange.xml
  2. EGL_KHR_image_base http://www.khronos.org/registry/egl/extensions/KHR/EGL_KHR_image_base.txt
  3. GL_OES_EGL_image http://www.khronos.org/registry/gles/extensions/OES/OES_EGL_image.txt
  4. Using direct textures on Android http://snorp.net/2011/12/16/android-direct-texture.html
  5. Using OpenGL ES to Accelerate Apps with Legacy 2D GUIs http://software.intel.com/en-us/articles/using-opengl-es-to-accelerate-apps-with-legacy-2d-guis
  6. iOS solution http://stackoverflow.com/questions/9550297/faster-alternative-to-glreadpixels-in-iphone-opengl-es-2-0