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Provide a matrix inverse shim for GLES 2.0. #50545

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Feb 15, 2024
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Provide a matrix inverse shim for GLES 2.0. #50545

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5 changes: 3 additions & 2 deletions impeller/compiler/shader_lib/impeller/gradient.glsl
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Original file line number Diff line number Diff line change
Expand Up @@ -6,13 +6,14 @@
#define GRADIENT_GLSL_

#include <impeller/texture.glsl>
#include <impeller/transform.glsl>

mat3 IPMapToUnitX(vec2 p0, vec2 p1) {
// Returns a matrix that maps [p0, p1] to [(0, 0), (1, 0)]. Results are
// undefined if p0 = p1.
return mat3(0.0, -1.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0) *
inverse(mat3(p1.y - p0.y, p0.x - p1.x, 0.0, p1.x - p0.x, p1.y - p0.y,
0.0, p0.x, p0.y, 1.0));
IPMat3Inverse(mat3(p1.y - p0.y, p0.x - p1.x, 0.0, p1.x - p0.x,
p1.y - p0.y, 0.0, p0.x, p0.y, 1.0));
}

/// Compute the t value for a conical gradient at point `p` between the 2
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31 changes: 30 additions & 1 deletion impeller/compiler/shader_lib/impeller/transform.glsl
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Expand Up @@ -26,4 +26,33 @@ vec4 IPPositionForGlyphPosition(mat4 mvp,
unit_position.y * destination_size.y, 0.0, 1.0);
}

#endif
#ifdef IMPELLER_TARGET_OPENGLES
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@jonahwilliams jonahwilliams Feb 11, 2024

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Its not possible to leave a define unevaluated with our shader compiler pipeline. that is, any define must evaluate to a value during the engine build - so its only possible to use these to turn this on/off for an entire backend.

If there is a runtime capabilities check you'd need to use, you'll have to use a specialization constant - which must be defined in the shader entrypoint and provided during initialization in content_context.cc

For an example of that, grep for supports_decal.

In this particular case, that might not be worthwhile.

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@johnoneil johnoneil Feb 13, 2024

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Thank you Jonah. I'm in a lot of meetings this week but will try to update this when possible.

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@bdero bdero Feb 13, 2024

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I don't think adding a second pipeline is worth the reduced flops of the O(1) invocation of this in conical gradient for on GLES only.

inverse isn't supported until GLSL ES 3. This restores GLES 2 support (GLSL ES 1).

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@bdero bdero Feb 14, 2024

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In other words: I think this approach seems OK as-is for now?

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@jonahwilliams jonahwilliams Feb 14, 2024

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Yes I agree, one ifdef for GLES


// Shim matrix `inverse` for versions that lack it.
// TODO: This could be gated on GLSL < 1.4.
mat3 IPMat3Inverse(mat3 m) {
float a00 = m[0][0], a01 = m[0][1], a02 = m[0][2];
float a10 = m[1][0], a11 = m[1][1], a12 = m[1][2];
float a20 = m[2][0], a21 = m[2][1], a22 = m[2][2];

float b01 = a22 * a11 - a12 * a21;
float b11 = -a22 * a10 + a12 * a20;
float b21 = a21 * a10 - a11 * a20;

float det = a00 * b01 + a01 * b11 + a02 * b21;

return mat3(b01, (-a22 * a01 + a02 * a21), (a12 * a01 - a02 * a11), b11,
(a22 * a00 - a02 * a20), (-a12 * a00 + a02 * a10), b21,
(-a21 * a00 + a01 * a20), (a11 * a00 - a01 * a10)) /
det;
}

#else // IMPELLER_TARGET_OPENGLES

mat3 IPMat3Inverse(mat3 m) {
return inverse(m);
}

#endif // IMPELLER_TARGET_OPENGLES

#endif // TRANSFORM_GLSL_