After the fix, the application retains the graph coordinates as logical data and passes adjacent point pairs to VA_Line for display. Drawing becomes an output of the calculation, not the source of coordinates used by control logic.
Where does the graph data path stop?
Follow the coordinate from its producer to the PPC2200 display. Application logic first acquires or calculates a point. A graph library converts that point into screen coordinates, selects two adjacent points as a line segment, and calls the VISAPI function VA_Line. The function receives the segment's starting X/Y coordinates, ending X/Y coordinates, and color palette index.
| Path stage | Input | Output | Failure visible at this stage |
|---|---|---|---|
| Application logic | Process values or operator-defined data | Logical coordinate set | Values are missing, invalid, or out of range |
| Graph library | Logical coordinates and graph geometry | Screen X/Y coordinates | Points are shifted, inverted, scaled incorrectly, or clipped |
| VISAPI drawing call | Two screen points and a palette index | One rendered line segment | No segment appears or the color is wrong |
| PPC2200 display | Rendered graph and background | Visible trace | The background, trace, or refresh behavior obscures the result |
VA_Line is the rendering boundary. It draws a segment; it does not calculate the process result or turn a visible trace back into logical coordinates. If later logic needs the points, store them before drawing and let both the control calculation and the renderer read the same stored data.
Is the requirement drawing, coordinate entry, or both?
Take the first reading at the application boundary: identify where each coordinate originates. This separates three different requirements that otherwise look like one graphing problem.
| Observed requirement | Coordinate source | Required implementation | Next check |
|---|---|---|---|
| Display calculated values | Application or process logic | Scale stored values and draw line segments | Validate the coordinate set |
| Use displayed points in later logic | The same stored coordinate set | Keep data independent of pixels; render a copy | Validate data-to-screen mapping |
| Let an operator draw or select points | Operator position input | Capture input coordinates, validate them, then render them | Check the input-to-graph transformation |
If coordinates already exist in logic, proceed directly to range and mapping checks. If the only existing artifact is a visible line, stop: screen pixels are not a dependable control-data store. Redrawing, clipping, background changes, and palette choices can change the image without changing the intended logical values.
Are the logical coordinates valid before rendering?
Read the coordinate collection before calling VISAPI. Every point used by a logical operation must have a defined origin, range, ordering rule, and invalid-data policy. Check these properties in the data layer, where a bad value can be distinguished from a drawing fault.
- Confirm that the collection contains enough points to form the intended segments. A polyline requires each point to be paired with the next point.
- Check that X values follow the intended ordering. Repeated or decreasing X values may be valid, but the resulting vertical or backward segment must be intentional.
- Compare each logical X and Y value with the configured graph ranges. Decide whether an out-of-range point is rejected, limited to the boundary, or omitted.
- Run the logical operation directly against the stored coordinates. Its result must not depend on whether the graph is currently visible.
If the stored values are wrong, repair acquisition or calculation before testing the display. If they are correct but the trace is wrong, continue to the coordinate transformation. This branch prevents a rendering change from masking a control-logic defect.
Does the data-to-screen transformation match the graph area?
Layer one first: identify the actual drawable rectangle on the PPC2200 screen and compare it with the background image. The graph library needs the rectangle's left, right, top, and bottom boundaries plus the logical minimum and maximum for each axis. Read those settings from the project rather than assuming that the full display is available.
| Reading | Expected relationship | Meaning when it fails |
|---|---|---|
| Logical minimum point | Maps to the configured graph boundary | Offset or scale is incorrect |
| Logical maximum point | Maps to the opposite graph boundary | Range, graph size, or endpoint handling is incorrect |
| Midrange point | Appears proportionally between the boundaries | Transformation is nonlinear or uses the wrong range |
| Increasing Y test | Moves in the intended visual direction | The vertical axis direction is reversed |
| Segment at an edge | Stays inside the intended plot area | Clipping or boundary policy is missing |
Use one transformation for every point. Do not let the logical calculation consume scaled pixel coordinates unless pixels are explicitly the required units. For operator-selected points, apply the inverse transformation before storing them so that control logic receives graph-domain values rather than display positions.
How should the VC4 graph library draw the trace?
Place graph rendering in a reusable user library. Supply a background image containing static elements such as the plot field, axes, or labels, then draw the changing trace over that background. This keeps graphic layout separate from coordinate storage and segment generation.
- Define the logical coordinate collection and retain it outside the drawing routine.
- Define the graph rectangle and logical axis ranges in the library configuration.
- Transform every valid logical point into a screen X/Y point.
- For each pair of adjacent screen points, call
VA_Linewith the first point, second point, and selected color palette index. - Apply the chosen invalid-point and out-of-range policy before issuing the drawing call.
- When refreshing the graph, restore or redraw the background as required by the screen design, then render the complete current trace.
Keep the call wrapper independent of the calculation. The renderer should accept points and draw them; it should not modify the values later consumed by logical operations. That separation also makes a blank graph diagnosable: inspect stored values, transformed pixels, segment endpoints, and the palette selection in that order.
How is the resolving branch verified?
Test with a small deterministic coordinate set whose expected positions are obvious from the configured axes. Include the logical minimum, midpoint, maximum, and at least one point at a graph boundary.
- Record the logical X/Y values before rendering.
- Record the transformed screen X/Y values passed to the drawing wrapper.
- Confirm that each adjacent pair produces one
VA_Linerequest with the intended color palette index. - Compare the visible endpoints with the matching locations on the background image.
- Run the logical operation and confirm that it reads the original logical coordinates, not the transformed pixels.
- Trigger a redraw and verify that the same data reproduces the same trace without changing the logical result.
FAQ
What happens if VC4 draws nothing with VA_Line?
Trace one segment through the path: validate its stored points, transformed screen endpoints, and color palette index. If those readings are correct, check whether the endpoints lie inside the configured graph rectangle and whether the background refresh removes the trace.
What happens if the VC4 graph is shifted or scaled incorrectly?
Compare the logical minimum, midpoint, and maximum with their expected screen positions. A common fault is using the wrong graph boundaries or logical range in the coordinate transformation.
What happens if increasing values move downward?
The logical Y direction and display Y direction differ. Correct the Y transformation in the graph library while leaving the stored logical coordinates unchanged.
Can VA_Line coordinates be used directly for logical operations?
Use the original logical coordinates for calculations and treat the VA_Line endpoints as display data. As the final verification, redraw the graph and confirm that the trace repeats while the stored coordinates and calculated result remain unchanged.