iOS: GPUImage Library and VBO - ios

Question

I am making use of Brad Larson's wonderful GPUImage library for image manipulation. So far, it's been great. However, I'm trying to add a filter to allow Mesh Deformation and running into quite a bit of issues. Specifically, I want to have a filter that uses VBO to render the Quad so I can ultimately dynamically change the vertices for the deformation.
The first step of using VBOs is causing a crash.
I created a subclass of GPUImageFilter overriding the - (void)newFrameReadyAtTime:(CMTime)frameTime method to render a quad via VBO. NOTE: I am simply trying to render a single Quad rather than a full mesh, that way I can tackle one issue at a time.
#implementation GPUMeshImageFilter {
GLuint _positionVBO;
GLuint _texcoordVBO;
GLuint _indexVBO;
BOOL isSetup_;
}
- (void)setupBuffers
{
static const GLsizeiptr verticesSize = 4 * 2 * sizeof(GLfloat);
static const GLfloat squareVertices[] = {
-1.0f, -1.0f,
1.0f, -1.0f,
-1.0f, 1.0f,
1.0f, 1.0f,
};
static const GLsizeiptr textureSize = 4 * 2 * sizeof(GLfloat);
static const GLfloat squareTextureCoordinates[] = {
0.0f, 0.0f,
1.0f, 0.0f,
0.0f, 1.0f,
1.0f, 1.0f,
};
static const GLsizeiptr indexSize = 4 * sizeof(GLushort);
static const GLushort indices[] = {
0,1,2,3,
};
glGenBuffers(1, &_indexVBO);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, _indexVBO);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, indexSize, indices, GL_STATIC_DRAW);
glGenBuffers(1, &_positionVBO);
glBindBuffer(GL_ARRAY_BUFFER, _positionVBO);
glBufferData(GL_ARRAY_BUFFER, verticesSize, squareVertices, GL_STATIC_DRAW);
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 2*sizeof(GLfloat), 0);
glGenBuffers(1, &_texcoordVBO);
glBindBuffer(GL_ARRAY_BUFFER, _texcoordVBO);
glBufferData(GL_ARRAY_BUFFER, textureSize, squareTextureCoordinates, GL_DYNAMIC_DRAW);
glEnableVertexAttribArray(1);
glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, 2*sizeof(GLfloat), 0);
NSLog(#"Setup complete");
}
- (void)newFrameReadyAtTime:(CMTime)frameTime;
{
if (!isSetup_) {
[self setupBuffers];
isSetup_ = YES;
}
if (self.preventRendering)
{
return;
}
[GPUImageOpenGLESContext useImageProcessingContext];
[self setFilterFBO];
[filterProgram use];
glClearColor(backgroundColorRed, backgroundColorGreen, backgroundColorBlue, backgroundColorAlpha);
glClear(GL_COLOR_BUFFER_BIT);
glActiveTexture(GL_TEXTURE2);
glBindTexture(GL_TEXTURE_2D, filterSourceTexture);
glUniform1i(filterInputTextureUniform, 2);
if (filterSourceTexture2 != 0)
{
glActiveTexture(GL_TEXTURE3);
glBindTexture(GL_TEXTURE_2D, filterSourceTexture2);
glUniform1i(filterInputTextureUniform2, 3);
}
NSLog(#"Draw VBO");
glDrawElements(GL_TRIANGLE_STRIP, 4, GL_UNSIGNED_SHORT, 0);
[self informTargetsAboutNewFrameAtTime:frameTime];
}
#end
Plugging in this filter, I see: "Setup complete" and "Draw VBO" displayed to the console. However, after it calls the target (in this case a GPUImageView) it crashes at the target's drawing call, which uses glDrawArrays.
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
Here is the complete method that contains this line.
- (void)newFrameReadyAtTime:(CMTime)frameTime;
{
[GPUImageOpenGLESContext useImageProcessingContext];
[self setDisplayFramebuffer];
[displayProgram use];
glClearColor(backgroundColorRed, backgroundColorGreen, backgroundColorBlue, backgroundColorAlpha);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
static const GLfloat textureCoordinates[] = {
0.0f, 1.0f,
1.0f, 1.0f,
0.0f, 0.0f,
1.0f, 0.0f,
};
glActiveTexture(GL_TEXTURE4);
glBindTexture(GL_TEXTURE_2D, inputTextureForDisplay);
glUniform1i(displayInputTextureUniform, 4);
glVertexAttribPointer(displayPositionAttribute, 2, GL_FLOAT, 0, 0, imageVertices);
glVertexAttribPointer(displayTextureCoordinateAttribute, 2, GL_FLOAT, 0, 0, textureCoordinates);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
[self presentFramebuffer];
}
Any help would be greatly appreciated, I've been banging my head against this for awhile.

Answer 1

It looks likely that the crash occurs because the GL_ARRAY_BUFFER is still bound when GPUImageView-newFrameReadyAtTime: executes.
Try unbinding the buffer (i.e. binding it to 0) at the end of -setupBuffers:
glBindBuffer(GL_ARRAY_BUFFER, 0);
The reason this is a problem is because GPUImage uses the same OpenGL context from one GPUImageInput (e.g. GPUImageFilter, GPUImageView) to the next. I believe largely in order that each step can output to an OpenGL texture and then have that texture directly available to the next GPUImageInput.
So because GL_ARRAY_BUFFER is still bound the behavior of glVertexAttribPointer inside GPUImageView-newFrameReadyAtTime changes, effectively trying point the displayPositionAttribute attribute to the populated VBO at an offset of imageVertices, which is nonsensical and likely to cause a crash. See the glVertexAttribPointer docs.

Answer 2

This code below doesn't look right to me at all. Why are you enabling vertex attrib array 4 & 5? You should enable the array at the attribute location you are intending to use.
//position vbo
glEnableVertexAttribArray(4);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 2*sizeof(GLfloat), 0);
//texcoord vbo
glEnableVertexAttribArray(5);
glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, 2*sizeof(GLfloat), 0);
If your vertex attribute is at position 0, you should enable attrib 0 and set pointer for attrib 0. If it's at position 4 (which I doubt), then you should enable attrib 4 and set the pointer for position 4. I can't think of any reason it should be mismatched like you have it.
You should either get the proper position by either setting it via a layout attribute, using glBindAttribLocation before shader linking, or using glGetAttribLocation after linking.
Let me know if this doesn't make sense.