iphone openGL ES 2 translating object

Question

looking to move objects in scene with matrix translation, but can't get anything to stick. I can get the projection to move but i cant get the 2d square to move.

Render code in sprite class:

glGenBuffers(1, &self->_vertexBuffer);
glBindBuffer(GL_ARRAY_BUFFER, self.vertexBuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(Vertices), Vertices, GL_DYNAMIC_DRAW);



glEnableVertexAttribArray(self.shader.positionSlot);
glVertexAttribPointer(self.shader.positionSlot, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), 0);

glEnableVertexAttribArray(self.shader.colorSlot);
glVertexAttribPointer(self.shader.colorSlot, 4, GL_FLOAT, GL_FALSE, sizeof(Vertex), (GLvoid *)(sizeof(float) * 3));

GLuint indexBuffer;
glGenBuffers(1, &indexBuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBuffer);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(Indices), Indices, GL_STATIC_DRAW);

float matrix[16]; // trying to manipulate matrix here 
matrixSetIdentity(matrix);

float translateMatrix[16];
matrixSetTranslation(translateMatrix, 0, 0, 0);

float result[16];
matrixMultiply(matrix, translateMatrix, result);
glUniformMatrix4fv(self.shader.positionSlot, 1, 0, result); //this doesn't work, the object disappears?

glDrawArrays(GL_TRIANGLES, 0, sizeof(Vertices)/sizeof(Vertices[0]));
glDrawElements(GL_TRIANGLES, sizeof(Indices)/sizeof(Indices[0]), GL_UNSIGNED_BYTE, 0);

heres the shader code:

attribute vec4 Position;
attribute vec4 SourceColor;

varying vec4 DestinationColor;
uniform mat4 Projection;
uniform mat4 Modelview;


 void main(void) {
 DestinationColor = SourceColor;
 gl_Position = Projection * Modelview * Position;
}

Now the frustrating part is that this works:

float h = 4.0f * self.frame.size.height / self.frame.size.width;
float projection[16];
[self.matrix projection:projection Left:-2 Right:2 Top:h/2 Bottom:-h/2 near:4 far:20];
glUniformMatrix4fv(self.shader.projectionUniform, 1, 0, projection);

float modelView[16];

matrixSetTranslation(modelView, 0.0, sin(CACurrentMediaTime()), -10.0);
glUniformMatrix4fv(self.shader.modelViewUniform, 1, 0, modelView);

glViewport(0, 0, self.frame.size.width, self.frame.size.height);

not sure why the top code doesn't seem to work but the bottom does. Any comments are greatly appreciated.

Answer 1

The projection matrix is being translated by a value which changes every frame (CACurrentMediaTime()). The model's transformation matrix is being translated by the same value.

That value is a translation of 0 x,0 y, and 0 z units (assuming that matrixSetTranslation behaves like GLKMatrixMakeTranslation ( https://developer.apple.com/library/ios/documentation/GLkit/Reference/GLKMatrix4/Reference/reference.html#//apple_ref/c/func/GLKMatrix4MakeTranslation )), which is the identity matrix, multiplied by the identity matrix again. This give you the identity matrix, which is equivalent to multiplying a regular number by one.