[vlc-devel] [PATCH] opengl: use common fragment shader for Android

[zhilizhao]

> On Dec 19, 2019, at 8:58 PM, Romain Vimont <rom1v at videolabs.io> wrote:
> 
> On Thu, Dec 19, 2019 at 08:15:23PM +0800, zhilizhao wrote:
>> Hi Romain,
>> 
>>> On Dec 19, 2019, at 6:32 PM, Romain Vimont <rom1v at videolabs.io> wrote:
>>> @@ -315,12 +322,13 @@ static GLuint BuildVertexShader(const opengl_tex_converter_t *tc,
>>>        "attribute vec4 MultiTexCoord0;\n"
>>>        "%s%s"
>>>        "attribute vec3 VertexPosition;\n"
>>> +        "uniform mat4 TransformMatrix;\n"
>>>        "uniform mat4 OrientationMatrix;\n"
>>>        "uniform mat4 ProjectionMatrix;\n"
>>>        "uniform mat4 ZoomMatrix;\n"
>>>        "uniform mat4 ViewMatrix;\n"
>>>        "void main() {\n"
>>> -        " TexCoord0 = vec4(OrientationMatrix * MultiTexCoord0).st;\n"
>>> +        " TexCoord0 = vec4(TransformMatrix * OrientationMatrix * MultiTexCoord0).st;\n”
>> 
>> TransformMatrix requires the texture coordinate has the form of (s, t, 0, 1). Does the condition satisfied in this case?
> 
> Good question :)
> 
> Yes, I think it is always satisfied.
> 
> Given that MultiTexCoord0 represent (2D) texture coordinates, it is
> always in the form [x, y, 0, 1].
> 
> OrientationMatrix is initialized in getOrientationTransformMatrix(). In
> this function, the matrix is first initialized to identity _(oh, there
> was already an "identity" constant, I will reuse it)_, then some
> components are overwritten. Note that in memory, it is stored in
> column-major order, so indices initialization must be intrepreted as:
> 
>    matrix[col * 4 + row] = value;
> 
> We see that row 3 is never overwritten, so the last row of the
> orientation matrix is necessarily [0, 0, 0, 1], and the last component
> of the output is 1.
> 
> Since MultiTexCoord0[2] == 0 (2D input coordinates, z == 0), row 2 is
> irrelevant, and the third component of the output is 0.
> 
> Therefore, OrientationMatrix * MultiTexCoord0 is always in the form
> [x', y', 0, 1] (which is the input of the TransformMatrix).
> 
> More intuitively, OrientationMatrix only represent 2D linear
> transformations (rotation, flip, transpose…) of 2D vectors (texture
> coordinates).

Thank you for such comprehensive explanation! I forgot matrix in
OpenGL is column-major by default, every time. TransformMatrix
on Android is basically do the same thing as orientationMatrix, plus
crop.

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