Pipeline: support +omit_fwd and +omit_inv keywords

Inspired from syntax of https://github.com/OSGeo/PROJ/pull/453/files
but 'rebased' on top of previous commit that cleans up the pipeline implementation

Different situations:
- +omit_fwd:
    the step when followed in the forward path will be omitted
    the step when followed in the reverse path will be executed
- +omit_fwd +inv:
    the step when followed in the forward path will be omitted
    the step when followed in the reverse path will be executed (with the inv method)
- +omit_inv:
    the step when followed in the forward path will be executed
    the step when followed in the reverse path will be omitted
- +omit_inv +inv:
    the step when followed in the forward path will be executed (with the inv method)
    the step when followed in the reverse path will be omitted

This will be used in the next commit to optimize constructs like

+step +proj=hgridshift +grids=foo
+step +proj=vgridshift +grids=bar
+step +inv +proj=hgridshift +grids=foo

Such steps are used for CRS to CRS transformations where applying the vertical grid
requires to do a transformation to an interpolating CRS. One can notice that
in the last step will just restore the horizontal coordinates before the first step, so
doing an inverse hgridshift is overkill.

So that could be optimized as:

+step +proj=push +v_1 +v_2
+step +proj=hgridshift +grids=foo +omit_inv
+step +proj=vgridshift +grids=bar
+step +inv +proj=hgridshift +grids=foo +omit_fwd
+step +proj=pop +v_1 +v_2

In the forward path, this will be equivalent to:
+step +proj=push +v_1 +v_2
+step +proj=hgridshift +grids=foo
+step +proj=vgridshift +grids=bar
+step +prop=pop +v_1 +v_2

And similarly in the reverse path, this will be quivalent to:
+step +proj=push +v_1 +v_2
+step +proj=hgridshift +grids=foo
+step +inv +proj=vgridshift +grids=bar
+step +proj=pop +v_1 +v_2

2 files changed, 58 insertions, 12 deletions

diff --git a/src/iso19111/io.cpp b/src/iso19111/io.cpp
index 678c5d98..d994277b 100644
--- a/src/iso19111/io.cpp
+++ b/src/iso19111/io.cpp
@@ -6284,6 +6284,15 @@ struct Step {
     };
 
     std::vector<KeyValue> paramValues{};
+
+    bool hasKey(const char *keyName) const {
+        for (const auto &kv : paramValues) {
+            if (kv.key == keyName) {
+                return true;
+            }
+        }
+        return false;
+    }
 };
 
 Step::KeyValue::KeyValue(const char *keyIn, const std::string &valueIn)
@@ -6813,7 +6822,9 @@ const std::string &PROJStringFormatter::toString() const {
 
     if (d->steps_.size() > 1 ||
         (d->steps_.size() == 1 &&
-         (d->steps_.front().inverted || !d->globalParamValues_.empty()))) {
+         (d->steps_.front().inverted || d->steps_.front().hasKey("omit_inv") ||
+          d->steps_.front().hasKey("omit_fwd") ||
+          !d->globalParamValues_.empty()))) {
         d->appendToResult("+proj=pipeline");
 
         for (const auto &paramValue : d->globalParamValues_) {
diff --git a/src/pipeline.cpp b/src/pipeline.cpp
index bc6708b0..99b2bdfb 100644
--- a/src/pipeline.cpp
+++ b/src/pipeline.cpp
@@ -116,9 +116,14 @@ namespace { // anonymous namespace
 
 struct Step {
     PJ* pj = nullptr;
-
-    Step(PJ* pjIn): pj(pjIn) {}
-    Step(Step&& other): pj(std::move(other.pj)) { other.pj = nullptr; }
+    bool omit_fwd = false;
+    bool omit_inv = false;
+
+    Step(PJ* pjIn, bool omitFwdIn, bool omitInvIn):
+        pj(pjIn), omit_fwd(omitFwdIn), omit_inv(omitInvIn) {}
+    Step(Step&& other): pj(std::move(other.pj)),
+                        omit_fwd(other.omit_fwd),
+                        omit_inv(other.omit_inv) { other.pj = nullptr; }
     Step(const Step&) = delete;
     Step& operator=(const Step&) = delete;
 
@@ -161,7 +166,12 @@ void pj_pipeline_assign_context_to_steps( PJ* P, PJ_CONTEXT* ctx )
 static PJ_COORD pipeline_forward_4d (PJ_COORD point, PJ *P) {
     auto opaque = static_cast<struct pj_opaque*>(P->opaque);
     for( auto& step: opaque->steps )
-         point = proj_trans (step.pj, PJ_FWD, point);
+    {
+        if( !step.omit_fwd )
+        {
+            point = proj_trans (step.pj, PJ_FWD, point);
+        }
+    }
 
     return point;
 }
@@ -171,7 +181,11 @@ static PJ_COORD pipeline_reverse_4d (PJ_COORD point, PJ *P) {
     auto opaque = static_cast<struct pj_opaque*>(P->opaque);
     for( auto it = opaque->steps.rbegin(); it != opaque->steps.rend(); ++it )
     {
-        point = proj_trans (it->pj, PJ_INV, point);
+        const auto& step = *it;
+        if( !step.omit_inv )
+        {
+            point = proj_trans (step.pj, PJ_INV, point);
+        }
     }
 
     return point;
@@ -185,7 +199,12 @@ static PJ_XYZ pipeline_forward_3d (PJ_LPZ lpz, PJ *P) {
     point.lpz = lpz;
     auto opaque = static_cast<struct pj_opaque*>(P->opaque);
     for( auto& step: opaque->steps )
-         point = pj_approx_3D_trans (step.pj, PJ_FWD, point);
+    {
+        if( !step.omit_fwd )
+        {
+            point = pj_approx_3D_trans (step.pj, PJ_FWD, point);
+        }
+    }
 
     return point.xyz;
 }
@@ -197,7 +216,11 @@ static PJ_LPZ pipeline_reverse_3d (PJ_XYZ xyz, PJ *P) {
     auto opaque = static_cast<struct pj_opaque*>(P->opaque);
     for( auto it = opaque->steps.rbegin(); it != opaque->steps.rend(); ++it )
     {
-        point = proj_trans (it->pj, PJ_INV, point);
+        const auto& step = *it;
+        if( !step.omit_inv )
+        {
+            point = proj_trans (step.pj, PJ_INV, point);
+        }
     }
 
     return point.lpz;
@@ -211,7 +234,12 @@ static PJ_XY pipeline_forward (PJ_LP lp, PJ *P) {
     point.lp = lp;
     auto opaque = static_cast<struct pj_opaque*>(P->opaque);
     for( auto& step: opaque->steps )
-         point = pj_approx_2D_trans (step.pj, PJ_FWD, point);
+    {
+        if( !step.omit_fwd )
+        {
+            point = pj_approx_2D_trans (step.pj, PJ_FWD, point);
+        }
+    }
 
     return point.xy;
 }
@@ -223,7 +251,11 @@ static PJ_LP pipeline_reverse (PJ_XY xy, PJ *P) {
     auto opaque = static_cast<struct pj_opaque*>(P->opaque);
     for( auto it = opaque->steps.rbegin(); it != opaque->steps.rend(); ++it )
     {
-        point = pj_approx_2D_trans (it->pj, PJ_INV, point);
+        const auto& step = *it;
+        if( !step.omit_inv )
+        {
+            point = pj_approx_2D_trans (step.pj, PJ_INV, point);
+        }
     }
 
     return point.lp;
@@ -476,13 +508,16 @@ PJ *OPERATION(pipeline,0) {
         proj_errno_restore (P, err);
 
         /* Is this step inverted? */
-        for (j = 0;  j < current_argc; j++)
+        for (j = 0;  j < current_argc; j++) {
             if (0==strcmp("inv", current_argv[j])) {
                 /* if +inv exists in both global and local args the forward operation should be used */
                 next_step->inverted = next_step->inverted == 0 ? 1 : 0;
             }
+        }
 
-        opaque->steps.emplace_back(next_step);
+        bool omit_fwd = pj_param(P->ctx, next_step->params, "bomit_fwd").i != 0;
+        bool omit_inv = pj_param(P->ctx, next_step->params, "bomit_inv").i != 0;
+        opaque->steps.emplace_back(next_step, omit_fwd, omit_inv);
 
         proj_log_trace (P, "Pipeline at [%p]:    step at [%p] (%s) done", P, next_step, current_argv[0]);
     }