[vlc-devel] [PATCH] opengl: use common fragment shader for Android
Romain Vimont
rom1v at videolabs.io
Thu Dec 19 13:58:10 CET 2019
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).
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